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My mx record for example.com points to mail.example.com I have only set an AAAA record for mail.example.com, but no A record. I don't want to expose my IPv4, only the IPv6.

I can receive e-mail from a Gmail account, but when I run tests with online "send anonymous email" tools, the mails don't show up (but the same tools do work with my Gmail account).

When I use mxtoolbox, I see the correct IPv6 as well as a {No A Record} message.

Is it recommended to have an A-record set up, or should having only an AAAA-record work fine as well? What else could be the problem?

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With only an AAAA record your server is only available via IPv6. I couldn't find much statistics on email server IPv6 connectivity, but for example, as of March 2020, below 30% of Google users were IPv6 capable. Majority of the Internet still runs on IPv4, and you can't just drop support for it without consequences.

Moreover, your server's IPv4 address is not a secret information you should protect. Certainly it's not any special compared to exposing your IPv6 address: if someone has malicious intentions, that's not an obstacle. And as SMTP probably advertises the same mailname in SMTP banner on every interface anyway, it's bad at keeping such secrets.

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Many hosts still have no IPv6 connectivity, so they won't be able to reach your mail server. Until IPv6 is the norm and deployed everywhere, the lack of IPv4 address is going to be a problem every time a non-IPv6-capable machine needs to deliver mail to you.

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