gmail-smtp-in.l.google.com[2a00:1450:400c:c02::1b] said: 550-5.7.1 [abcd:ef01:23:45:67:8:9abc:def0] Our system has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records 550-5.7.1 and authentication. Please review 550-5.7.1
https://support.google.com/mail/?p=ipv6_authentication_error for more 550-5.7.1 information. u5si1912210wyu.71 - gsmtp (in reply to end of DATA
you’re probably using Postfix with IPv6 turned on. To fix this, tell Postfix to just use IPv4. In /etc/postfix/main.cf, change
inet_protocols = all
inet_protocols = ipv4
If it’s missing, just add the latter line. Restart postfix and hoppla! Es klingelt.
Gmail also wants a proper PTR (reverse IP lookup) record for the mail server, but the error message is probably something different (or nonexistent) if that’s missing.