Most legitimate mail senders know to process bounce messages. This means that if they get what is called a “hard bounce” for an email address on their list (meaning that the bounce tells them that the email address simply doesn’t exist), they remove the email address from their list permanently (and immediately, right guys?) If they get a soft bounce (such as “mailbox full”), they may sideline that email address and try it again in a week or so.

Failure to remove bounced email addresses, particularly hard-bounced addresses, can lead to and result in your email being blocked. That’s because repeatedly sending to non-existent email addresses is taken by most inbox providers and other ISPs as a sure sign that either a) you are sending to very old lists, and you don’t care, because you’re a spammer, or b) you are sending to a list that you ‘acquired’ from somewhere else, so you’re a spammer, or c) you’re a spammer.

So clearly processing email addresses that bounce and getting them off your list as quickly as possible is a Very Important Thing to Do. Again, most legitimate email senders know to do this, and so as soon as they get that email bounced back to them saying that the address does not exist, they remove the email address. [In addition to your deliverability taking a hit if you don’t process bounce messages, not realizing that an email address is undeliverable can actually be dangerous to someone’s safety.]

But..guess what.

Oh Where, Oh Where Have My Bounce Messages Gone?

The emailed bounce notification is generated by your own email system (i.e. your sending server), based on the rejection in your email logs. But sometimes you won’t get that bounce notification by email, whether because your system isn’t set up correctly, or your system failed, or some other reason, it doesn’t really matter, the point is that only your email logs will reflect that rejection (bounce). Which means that if you try to send an email to xy*@ex*****.com, and xy*@ex*****.com doesn’t exist, you will not get an email notification of your email bouncing. Instead, you will see something like this in your mail log:

550 <xy*@ex*****.com>, Recipient unknown

(It’s also super important that you understand those SMTP error codes in those bounce messages.)

Now, the upside to this when it happens, if it can be said that there is an upside, is that if a spammer sends out a spam run, and uses your return address in the “From:” field (making it look like the spam came from you), your email receiving system will not be crippled by a tsunami of email coming to you with the bounce messages, as can and does happen sometimes. As data protection service GalaxKey explains, “For the most part if a spammer sends out mail using your spoofed address and it is rejected by the recipient’s server, the emails will generally bounce back to your authentic address, filling your inbox with ‘unable to deliver messages’.”

But, it also means that you need to know to monitor your mail logs for these rejections, and to treat them just like any other bounce. Which means get that bouncing email address off your mailing list, asap.

Otherwise you’ll look like a ..well, you know..and your mail will get…you know.

ISPs Inserting Bounce Messages in Your Mail Logs and NOT Sending Them by Email

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3 Responses

  1. Bouncing in stream is how SMTP is supposed to work. The idea of the sender, say, sending a bounce after the SMTP stream is completed is non-standard.

    Every MTA works this way, and most ISPs do. AOL for years was the exception, doing out of stream bounces.

    If your sending MTA (or ESP) isn’t taking the in stream bounces and sending them to the envelope-from address then you need to upgrade.

  2. The sending mailserver is supposed to create a bounce email on a hardbounce (like a 550). Why should the receiving mailserver generate such an email if the sender (mailserver) already has been informed probably?

    This is not new or strange, just the same behavior it used to be. You’d get two bounce mails otherwise.

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