Honestly? Don't send to Gmail over IPv6

Yes, some people do it, and yes, for some people, it works fine. But.....

If you're standing up a new server and it has an IPv6 IP address, and it has no sending reputation, you'll find it very hard to get mail reliably delivered to Gmail inboxes. You're starting with two strikes against you -- both reputation and authentication requirements.

If you're going to try to do it -- start here:

  1. Make sure you have working forward and reverse DNS (for both IPv4 and IPv6, if you have both).
  2. Make sure you implement SPF.
  3. Make sure you implement DKIM.
  4. Maybe even implement DMARC.
  5. Start out very slow, very low volume, a few messages at a time. Do they go through? If not, maybe you've sent too many. Do not increase the limit per test until you stop seeing blocking.
  6. Give up and disable the IPv6 interface (or configure Postfix to route the mail out to Gmail via the IPv4 interface).
  7. Realize that this if solves all of your problems, maybe you should stop here, and crack open a cold, possibly alcoholic, beverage.

Ultimately, Gmail is very fussy about reputation and authentication, and even more so on IPv6. I've probably seen at least a half dozen complaints from hobbyist users this year that when setting up their new server, Gmail won't let their mail through, and it's almost always this issue. Is switching to IPv4 a perfect solution? No. But sometimes email isn't perfect. It depends on whether or not you want to fight it, or just get it done and over with.

If you're sure there ought to be another way -- or you want to ask other mail server administrators for suggestions, the Mailop list is where you do that. But I'm telling you now, if you're in a hurry to try to solve this, IPv6 is probably not going to be your friend here. 

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