SMTP codes can be so confusing, especially those SMTP response codes and SMTP error codes like the dreaded SMTP error 550. These are often returned in bounced email, such as “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site.” They are in response to SMTP commands that have ‘gone wrong’ between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. But what exactly do they mean? Why should you care and what should you do? And why do we say that SMTP error 550 is dreaded?

SMTP Error Codes and What They Mean

First, ‘SMTP’ stands for Simple Mail Transfer Protocol. It is the way that your sending email server connects with the email server on the other end.

Here is a general list of SMTP error codes, and what each message is supposed to say and mean:

211 – A system status message.
214 – A help message for a human reader follows.
220 – SMTP Service ready.
221 – Service closing.
250 – Requested action taken and completed.
251 – The recipient is not local to the server, but the server will accept and forward the message.
252 – The recipient cannot be VRFYed, but the server accepts the message and attempts delivery.
354 – Start message input and end with .. This indicates that the server is ready to accept the message itself
421 – The service is not available and the connection will be closed.
450 – The requested command failed because the user’s mailbox was unavailable (such as being full). Try again later.
451 – The command has been aborted due to a server error. (on their side)
452 – The command has been aborted because the server has insufficient system storage.

And, of course, the 500 series of SMTP error codes, which are the SMTP error codes to which you, as a general email sender, will most often see, and to which you need to pay attention. Unfortunately many receiving systems seem to mix and match these error messages, rather than adhering to the prescribed message code meaning. (IANA has a full comprehensive list of SMTP error codes and what they all mean.)

The good news is that the SMTP error messages you need to worry about as an email sender are really primarily limited to just a few (i.e. those in the 500+ range) and of those, the ones that you really need to worry about are the ones in the 55X range – i.e. 550-559. And of those, by far the most common one and the one that as a sender you will see most often and to which you must pay immediate attention is the 550 SMTP error code, which can range from “user not found” to “mailbox unavailable” to any number of other similar variations, but which for you, dear email sender, should almost always be read to mean “remove this email address from your mailing list immediately.”

(On the other hand, your mail server administrator should care deeply about many more of these codes, however we assume that if they are administering your outbound mail server, they are already familiar with them.)

So, back to that 550 error code. Almost always, when you get an email bounced back (or inserted in your mail log!) with a 550 error code, it means that the receiving system could not deliver your email to the user to whom it was addressed because the mailbox is unavailable. Almost always, this means that the inbox either no longer exists, or that it never existed!

Why would you have an email address on your mailing list that has never existed? There could be a lot of reasons, including that someone entered it wrong, or that someone intentionally entered a fake email address into your system (such as when you require a user to divulge an email address in order to receive a download, etc.).

Regardless of how it ended up on your mailing list, if you send a mailing to it, it tells the receiving system one sure thing: that you don’t confirm email addresses before adding them to your mailing lists.

And, if you continue to send email to that same non-existent email address after receiving the 550 message that the mailbox doesn’t exist, it tells that receiving system that not only don’t you confirm email addresses, but that you don’t care very much about list hygiene – i.e. that you don’t maintain your mailing lists according to best practices, either.

And, having determined this, very soon those receiving systems, including inbox providers and other ISPs, will simply stop delivering your email to the inbox – first diverting it to the junk folder, then not delivering it at all, and then perhaps even blocking all of your email.

Now, it’s possible that a mailbox will be unavailable because a user has let their inbox get full. And we often get asked whether, because of that possibility, it’s ok to not remove an email address which has returned a 550 error?

Think about this: in this day and age of nearly unlimited email storage (nearly all ISPs now offer multiple Gigs of email storage), just how inactive does a user have to be in order for their inbox to fill up? And, even if they are going to come back some day and clear out their inbox, do you think that they are really going to stop to read your email? Or will they delete it unopened and unread, creating even more problems for you?

The bottom line is: why hang on to an email address that belongs to a user who will never be a positive asset for you, and can only cause you trouble?

So the next time an email that you send bounces back, take a good look at the information in the bounce message, and take the appropriate action based on that message. And don’t forget to check your mail logs for bounce messages too!

P.S. If you are actually receiving bounce messages that include “mail from refused spam site” or a similar message, the odds are good that your mail already has been blocklisted, and you’ll need to deal with that immediately.

SMTP error codes 550

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53 Responses

  1. What about the 450 message? From what I know 4xx messages indicate a temporary failure, but isn’t a “full mailbox” also a temporary failure? A lot of MTA’s will immediately generate a bounce message for 550 but most will keep trying to deliver a message that fails with 450 for about a week ( or whatever the “queue life time” is for a certain server). If this considered harmful by the ISPs?
    Should we remove these email addresses ( that fail with a 450 error after trying for a week ) immediately from the list ( like we do with 550 codes ) or should we maybe send another mailing ?

    Some free email providers will deactivate an account if is inactive for X days. ( I think yahoo used to do this or still does ) . In this they would send a 450 message not a 550 message as the user can still reactivate his account by simply logging in.

  2. Hi there,
    we have an unusual 550 problem, when we send out mail via our web site to a specific email address we get a 550 mail bounced straight back to us, however if we use the same email address vai outlook (using the same smtp server to send it) then the messages get through without a problem? Any suggestions??

    The bounce we get is (with actual email address an IP’s removed):
    Your message did not reach some or all of the intended recipients.

    Sent: 19 Jan 2010 20:07:38 +0000
    Subject: Password Reset

    The following recipient(s) could not be reached:

    Error Type: SMTP
    Remote server ( issued an error.
    hMailServer sent: .
    Remote server replied: 550 xx*******@Ec******.com)

  3. Hi Reuben

    To answer your question, you will need to tell us the specific 550 error, usually it will tell you what has happened. Also check whether your mail servers are not appearing on any spam blacklists.

  4. Thanks for that exhaustive yet concise description which I have bookmarked. And while I did normally remove 550s I usually left the “over quota” guys in in the (as you explained quite vain) hope they might eventually clear up their mailbox and read the mails. No, you’re right, I probably wouldn’t either. But I wasn’t aware of the ramifications and that it would lower my “sender credit score” so to speak.

  5. I’m having a 550 problem that I have not been able to get a solution on for more than 3 years. The error I get is:

    Your message did not reach some or all of the intended recipients.

    Subject: FW: The Clinical Device Newsletter
    Sent: 6.5.2010 08:12

    The following recipient(s) could not be reached:

    aa***@bb***.bb’ on 6.5.2010 08:12
    550 Error: Lets just say we’re removing now.

    No one that I have been in contact with has any knowledge of this error. I have used Outlook 2003/2007, Eudora 8 and Thunderbird 2 and 3 and I allways get the same error sending HTML mail with pictures(these are newsletters). I use a POP account and my IT provider claims that this is not on his side, he has informed me that during this period he has changed mail-server hardware and software so that this should not be his fault. Can anyone help with this weird error??

  6. Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

  7. Hey Dude …(assuming you are a Dude)

    I give this article an A+

    Excellent with very useful information

    Thanks GC

  8. hey im having the same problem as TIN

    May 11, 2010 5:26
    Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

    recently ive been trying to contact someone as well an their email has worked jus fine for years and its like all of a sudden i cant get them on IM and i cant get them in email my mail bounces back and im not sure if maybe they just havent been on and it could be that their mailbox is just full. but this is the first time in years ive ever had a problem with this..please help!

  9. I think it can also be said that just because you have a 55X error message, doesn’t necessarily mean your list isn’t clean or that you don’t check your list for hygiene.

    Consider the case where a mail sender hasn’t e-mailed the list in longer than 30 days — it is quite possible to receive a 55X range error message in that case, because people close their e-mail accounts, etc quite frequently.

    That said, I think it can be argued (and in my own experience) most ISP’s don’t just automatically assume that you don’t confirm your e-mails simply because they issued you a 55X response code.

    Now of course, if you receive the same 55X error code for the same *exact* e-mail address a 2nd or a 3rd time, then yes, it can be assumed you don’t sanitize your mailing list, but simply receiving a 55X error code one time, doesn’t necessarily indicate that you don’t sanitize your list.

    This is especially true with mail providers such as Yahoo! and Gmail (in my own experience).

  10. hey im having the same problem as TIN

    Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

  11. Ramesh,

    Please bear in mind that 550 could also mean that the user’s mailbox is full.

    Different ISP’s respond differently in this regard, so if after a few tries (say 2-3 tries) you’re still not getting mail delivered, it’s probably safe to temporarily remove the subscriber from your list (try again in a subsequent mailing).

    Probably the mailing after your next one depending on the frequency of your mailings may be a good idea (for example, if you e-mail once per month, it’s probably safe to e-mail them in the month following your next mailing).

    If at that point you receive a 550 again, it’s probably safe to permanently remove the e-mail from your list for the following reasons:

    1) If the mailbox is full, it probably means the user is not actively reading/monitoring e-mail sent to that account in the first place.

    2) Something else could be wrong, but in any case, no point in mailing to a subscriber who is almost surely not receiving your messages anyway.

    Let me know how you get on. . .

  12. We do electronic billing. Have just received this message:
    “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.

    After sending successfully for the past few weeks, this just occurred today. Am I reading correctly that the recipient is telling me that we are sending “spam?”

  13. Richard,

    No probably not.

    The problem is on the recipient’s side, not yours:

    “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator”

    This means something is wrong on their end (assuming you pasted the error message in its entirety).

  14. hi
    I have The problem to conect mailserver
    title eror is :
    SMTP -> FROM SERVER:220 ESMTP MDaemon 10.0.5; Thu, 24 Nov 2011 09:36:30 +0330
    SMTP -> FROM SERVER: Hello, pleased to meet you 250-ETRN 250-AUTH=LOGIN 250-AUTH LOGIN CRAM-MD5 250-8BITMIME 250 SIZE
    SMTP -> FROM SERVER:250 , Sender ok
    SMTP -> FROM SERVER:550 pa****@ag********.org must check for new mail first
    SMTP -> ERROR: RCPT not accepted from server: 550 pa****@ag********.org must check for new mail first
    SMTP Error: The following recipients failed: q.********@gm***.com SMTP Error: The following recipients failed: q.********@gm***.com

    SMTP server error: pa****@ag********.org must check for new mail first

    plz help me .

  15. Very good article. However I would disagree about hasty removal of the email that returns the 550 error message because in many corporate and government organisations who are using their own servers, limiting Storage Space for employees is their policy. Many emails now contain photos, graphics which are huge MBs. If one doesnt delete them regularly, then the Mail Box simply is Full. Suggest to wait for a few days and deliver your email again or if it is someone you know, just call up on a phone.

  16. I am not sure if I agree with all that was said although I didn’t understand all that I read. However, the relevant parts are that… I just decided to begin using Outlook with the help of another Outlook user who has since left and with whom I usually contact by email.

    I am now unable to send email by Outlook which is where I received my first 550 error message. So I went to send an email through the original site and received the same error message.

    Please advise

    Now the parts I don’t agree with is about not ever checking email once the box is full (which happened to be a work email with very limited space).

    When do get a chance to check clean out the box then I do open each email and check for relevancy.

  17. Someone trying to send to us is getting the following bounce returned to them…

    There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.

    I cannot work out if this issue is at our end or theirs.

  18. I have emailed my best friend for years from my work email. All of a sudden, when I tried to reply to an email from her it was bounced back with a 550 5.1.1 user unknown message. I tried composing a new email, but it was also bounced back. And the weird thing is that while she is in my address book, her email address does not “auto finish” when I type in the first letter of her address like it usually does. I have no problem emailing her using my gmail, it is only in Outlook. She knows of no changes on her end and hers is the only address that I have noticed being bounced back. What can I do?

  19. I also am getting mail bounced back/rejected: Here is the error message. Technical details of permanent failure:
    Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Requested action not taken: mailbox unavailable (state 17).

    What does this mean? How can I resolve it……..NOW…lol Thanks

  20. Hi,

    I am also annoyed with 55X errors messages.

    1st, when member register with our website, an auto-generated email goes to him/her to confirm and even those emails are not reaching to their mailbox. They click on resend again and again but they never get the email to confirm their email.

    Example messages are:

    1. 550 Recipient Rejected: Access Denied: blocked by prs
    2. 550 5.1.1 : Recipient address rejected: User unknown
    3. 554 delivery error:dd This user doesn’t have a account
    4. 550 5.7.0 Blocked
    5. 554 5.7.1 SPAM or UCE is not accepted here

    And when I check reputation of my IT through Return path, i find it 97. SPF record is pass.

    2nd, when I send emails to DOI members (those who have already confirmed email address), I again receive even more bounce backs with similar messages.

    What could be a solution here?

  21. FYI, a full mailbox 550 can happen if a user is forwarding all their mail to another address and doesn’t realize they have a box filling up somewhere in cyberspace. That happened to me years ago until friends told me they had a problem with my address and I figured it out. You can’t simply assume they aren’t reading any of their email.

  22. if you have 550 message error its alsa possible its because your smtp server is blacklisted by the recepient mail server (spam protection)

    Its what happened to me

  23. Error sending email – 554 Transaction failed : Cannot send message due to possible abuse?


    My Outlook was working perfectly so far. But From today morning I am not able to send mails. I am getting error message as following…

    554 Transaction failed : Cannot send message due to possible abuse; please visit for more information

    Please help me to solve this. I have tried searching many sites. Everywhere they were explaining the error but no solution for it.


  24. I am having a similar issue and not able to fix it since past 48-72 hours. I am using a web hosting with unlimited storage.
    My primary domain name is (On which i am not able to receive emails) & on the same hosting i have another domain (from which i am able to send messages on any gmail or yahoo email but they usually end up in Junk mail).
    I have a gmail account aswell and tried sending email to my primary domain (resulted in 550 Error on gmail)

    1. I need to receive emails to my primary domain.
    2. I need to stop the sending emails to end up in Junk Mails.

    ::::::::::Error While Sending To Primary Domain From Gmail::::::::::
    Technical details of permanent failure:
    Google tried to deliver your message, but it was rejected by the server for the recipient domain by [].

    The error that the other server returned was:
    550-The mail server could not deliver mail to ad***@am*******.in. The account
    550-or domain may not exist, they may be blacklisted, or missing the proper dns
    550 entries..

  25. While sending e-mails following error occurs :-

    Reason: 550 Your IP is listed as spammer (

    What could be the issue . Please help.

  26. OK, we have a problem with error 550 when sending to a valid email address.
    I manage a number of on-line sites through 1and1, my personal domain is via 1and1 also.

    Using either the on-line email system or using a local email client through IMAP we get the following error message (real domain replaced with ‘abcdefg’)

    Message could not be sent to the following recipients: [“abcdefg, Simon & Gemma” ]
    (550 – 550-Requested action not taken: mailbox unavailable
    550 invalid DNS MX or A/AAAA resource record)

    If I use my personal email account to send to the same address the message is sent without problems.

    Can anyone elighten me ?

  27. Hi,

    My Outlook was working perfectly so far. But From today morning I am not able to send mails. I am getting error message as following…

    host []
    SMTP error from remote mail server after end of data:
    550 Message contained unsafe content

    Please help me to solve this. Please…


  28. Hello sir,

    As i replied mail i get bounce mail with error
    This message was created automatically by mail delivery software.
    A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

    sa****@pr****.com INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :SPF: is not allowed to send mail from

    ga***@pr****.com INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :SPF: is not allowed to send mail from

    please help me

    • Vir:

      Not sure if you’re still experiencing issues, but the error message you mentioned means that the domain you are sending e-mail from is not defined in the SPF record for the server you’re sending from (i.e.

  29. I’m sending out Holiday emails with a picture. I’m guessing that one or more of the sendees are not valid emails anymore. But it blocks ALL of the other recipients. How can I get around this?

  30. For more than 2 months now my emails remain stuck in the outbox (INCREDIMAIL)
    THE error code is 550 5. 5. 1 protocol error, is there any help available.

  31. PROTOCOL ERROR CODE 550 5. 5. 1 (incredimail) is preventing me from sending emails
    they do not leave the outbox, is there a solution?

    • Carol:

      For help from this page, you’d have to past the message it its entirety, not just a piece of the error message — everything is relevant in error messages.

      I realize it’s been a months since you’re post but if you’re still experiencing issues, please paste the ENTIRE error message here and I’ll see if I can help you figure it out. ;-)

  32. I am writing to su*****@we****.com
    I got “550 Ip frequency limited.”

    I read it as – “Go away – there is no way we have the manpower to provide a support team for 800 million customers – work it out yourself”

    Its a pity. Wechat is a great programme but when it goes loopy – you are …. on you own. If you rely on it, as 800million people probably do …. Oh well.

    • Jeremy:

      When you receive the error message “IP Frequency Limited” this likely means that you are being rate-limited from sending e-mails to (meaning you’ve sent to many e-mails to their system). Either send less e-mail or put more information in each subsequent e-mail so that you can get everything delivered i one fell swoop.

  33. I am getting the SMPT 500 error code on an email address that wants to receive our email, and we’ve gone back and forth with the reader having him white list our IP addresses, adding us to his safe senders, etc. and still to no avail. Can’t seem to convince him this is on his side and not ours. Any suggestions.

  34. I have been emailing this company for a number of years and most recently as this past February with no problems. Now all of a sudden I get the following message:

    MailEnable: Message could not be delivered to some recipients.
    The following recipient(s) could not be reached:

    Recipient: [SMTP:np*@ni*********.com]
    Reason: Remote SMTP Server Returned: 554 5.7.1 Service unavailable; Client host [] blocked using

    I assume that it is on their end as I receive the same message whether I send via my desk top or smart phone. I have called the company and they are getting other emails with no problem.

  35. A Client of mine gets this error message and I have no clue where to look for a solution.

    The error message is ( 550 is not allowed to
    send mail from

    The IP address is OK. The would not resolve to that IP. However, the IP Adress would not resolve to But is in SPF comment in DNS records for that site.

    Any ideas would be greatly appreciated.

  36. You’re not taking into consideration what the NSA (National Security Agency) does. I was in a thread with a friend for two days. Suddenly, my Email was designated as SPAM. I had to solve it by using another of my Email addresses so my friend would receive it. Then, she answered that Email using my original Email address.

    Also, an Email I sent to my partner using a particular Email address garnered a fatal error. We also checked it using just a test message, and that, too, garnered a fatal error. Now, I know my partner’s Email address. He’s with Outlook.

    I have a friend in Seattle, WA, and one in Spain who send me Emails virtually every day, but when I try to reply to either of these friends using the Email address to which they sent me their Email, I get a fatal error. So I have to send my reply to them both using a different Email address.

    Arlene Johnson

  37. Can somebody please help. I am getting the following message when sending e mails. SMTP ERROR 550 Failed to add recipient 5.11.M -RCPT: Max recipient list exceeded limit

  38. The real problem here is that the email address DOES exist. For some reason though, a local restaurant receives this message when sending us emails, even though all other email vendors work. We can’t figure out why.

  39. GMass sending using Amazon SES error after 400 emails sent out

    Connected to smtps://
    <> EHLO []
    << 250-8BITMIME
    << 250-SIZE 10485760
    <> MAIL FROM:
    <> RCPT TO:
    <> DATA
    << 354 End data with .
    >> From: de********@gm***.com
    >> Date: Tue, 13 Feb 2018 21:42:36 퍍
    >> Subject: SMTP test
    >> Message-Id:
    >> To: he***********@ya***.ca
    >> MIME-Version: 1.0
    >> Content-Type: text/Test message
    >> .
    <> RSET
    << 250 Ok
    ERROR: Transaction failed: Expected ';', got "message"

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