After connecting to IPv6 network mail sent to GMail bounced with an error:

SMTP error from remote mail server after end of data:
 host gmail-smtp-in.l.google.com [2a00:1450:4008:c01::1a]:
 550-5.7.1 [2a02:e840:10:6:218:51ff:fed7:bb9f 16] Our system has detected
 550-5.7.1 that this message does not meet IPv6 sending guidelines regarding PTR
 550-5.7.1 records and authentication. Please review
 550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more
 550 5.7.1 information.

Temporary fix for this problem – setup Exim to resolve only IPv4 for GMail:

dns_ipv4_lookup = gmail-smtp-in.l.google.com : *.gmail-smtp-in.l.google.com