I tried to send an email to gmail on my Debian server (with the Vesta panel installed). I received this error:

 SMTP error from remote mail server after end of data:
 host gmail-smtp-in.l.google.com [2a00:1450:400c:c01::1b]:
 550-5.7.1 [2001:41d0:8:e974::1] Our system has detected that this message does
 550-5.7.1 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

Thus, I need to add a PTR record for my ipv6 address. However, not all providers provide such an opportunity. The easiest way to solve this issue is by disabling ipv6 in the mail system.

First, I checked:

netstat -tulpn | grep :25
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      17919/exim4     
tcp        0      0 0.0.0.0:2525            0.0.0.0:*               LISTEN      17919/exim4     
tcp6       0      0 :::25                   :::*                    LISTEN      17919/exim4     
tcp6       0      0 :::2525                 :::*                    LISTEN      17919/exim4

Then I opened Exim configuration file:

nano /etc/exim4/exim4.conf.template

I put a ban on ipv6 (disable_ipv6 = true) before the begin acl section as shown below:

######################################################################
#                       ACL CONFIGURATION                            #
#         Specifies access control lists for incoming SMTP mail      #
######################################################################

# disable ipv6
disable_ipv6=true

begin acl

Then I saved the changes and restarted Exim:

/etc/init.d/exim4 restart

Checked:

netstat -tulpn | grep :25
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      18286/exim4     
tcp        0      0 0.0.0.0:2525            0.0.0.0:*               LISTEN      18286/exim4

all is ready!