If you have all of your authentication and unsub headers set up correctly, and Gmail is still rejecting email that you send from Microsoft (Outlook, Exchange, 365, etc.), here’s why. First, you’re not crazy and you’re not doing anything wrong. It’s real, it’s on the Gmail side, and there’s a reason.

The issue is hinted at in the rejection message that you receive from Google (likely over and over again), which looks something like this:

Error:
550 5.7.350 Remote server returned message detected as spam -> 550 5.7.1 [2a01:111:f403:200a::700 19] Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. For more information, go to; https://support.google.com/mail/answer/188131d75a77b69052e-43df56e1c6fsi16667051cf.749 – gsmtp
Message rejected by mx.google.com

Now, let’s look at that again, with that hint highlighted:

Error:
550 5.7.350 Remote server returned message detected as spam -> 550 5.7.1 [2a01:111:f403:200a::700 19] Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. For more information, go to; https://support.google.com/mail/answer/188131d75a77b69052e-43df56e1c6fsi16667051cf.749 – gsmtp
Message rejected by mx.google.com

Here’s the thing: that sending domain to which they are referring is not your sending domain; it’s the Microsoft domain through which you are sending it (for example outlook.com).

You see, Gmail believes that all (or at least most) email coming from Outlook servers over IPv6 IP addresses is spam. But don’t take our word for it, here it is directly from someone at Google / Gmail (we are intentionally not revealing their identity as we don’t want them to get into trouble):

“In the past Outlook was using IPV6 to send the emails, and all of the emails were marked as spam just because of the reputation. And due to that past interaction, whatever emails coming from Outlook IPV6, Gmail has detected them as spam and just to protect our end user, they are rejecting the emails. To get this resolved you need to contact Outlook support and you should inform them to provide the IPV4 then your email won’t get bounced as spam.”

Translation: A whole lot of spam is sent through the Outlook IPv6 space, and so Gmail started assuming that anything coming from Microsoft’s IPv6 space is more likely spam than not, and eventually it started not just putting email sent from that space in the spam folder, but outright rejecting it.

Unfortunately, the advice to contact Outlook support is so unhelpful as to be laughable, because of course front line Microsoft support has no idea how to help you send through IPv4 instead of IPv6 when you don’t even have a dedicated IP address with them.

So you will go around and around with them, eventually getting such responses as “make sure that your SPF is set up correctly” and “make sure that your DKIM is set up correctly,” which are not only unhelpful because you already did all that, but also because the issue isn’t with your domain reputation, it’s with the domain reputation of the Microsoft servers that are spewing out spam over IPv6 (think of all of the free Outlook accounts that are used by spammers).

Fortunately there is a way to force your email from Microsoft to be sent over (and only over) IPv4 when sending to Gmail accounts. It involves self-help (you have to set it up yourself) and it involves knowing exactly how to do that.

We can tell you how to do this, but we’re not going to put that information in this article both because we don’t want Microsoft to find this article and change it (at least not before they fix their issues that are causing Google to block their IPv6 space), and also because while we knew all of the above, the last couple of steps were shared with our CEO in a private conversation with a colleague. So, email us here for those steps (no, we’re not going to charge you, and if you need help with the steps we are going to point you to that colleague).

Gmail Rejecting Your Email from Microsoft? Here's Why and How to Fix It

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