Gmail no longer accepts even mail from addresses to which it sends mail itself?

Question

gmail

I DO NOT HAVE A MASS MAIL. I'm just replying to an incoming email. I get:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  [email protected]
    SMTP error from remote mail server after end of data:
    host gmail-smtp-in.l.google.com:
    550-5.7.1 Our system has detected that this message
    550-5.7.1 does not meet IPv6 sending guidelines regarding PTR records and
    550-5.7.1 authentication. Please review
    550-5.7.1  https://support.google.com/mail/?p=ipv6_authentication_error for more
    550 5.7.1 information. v68si27409125vke.179 - gsmtp

The link leads to a document titled "Bulk Senders Guidelines", but what does that have to do with me? Should a whitelist work for an address from contacts? Or has he gone completely mad?
And what does IPv6 have to do with it? I have a normal white IPv4, but what does he not have?

Answer 1

Of course, I do not doubt your knowledge, but
the answer indicates that your IPv6 address does not have a PTR record (more precisely, it was not found in DNS)
This is necessary in order to determine your domain name by IP,
which in turn should point to your IP.
Is it possible that IPv6 is still enabled?
Is EXIM configured incorrectly?