This is how three years ago I solved IPv6 problems for all domains
that have Google as an MX host.
/etc/postfix/main.cf:
smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter
/etc/postfix/smtp_dns_reply_filter:
# /domain ttl IN AAAA address/ action, all case-insensitive.
# Note: the domain name ends in ".".
/^\S+\.google.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE
This drops all AAAA records from Google MX hosts, effectively forcing
Postfix to deliver over IPv4.
I adopted this because their PTR lookups did not distinguish between
'try again' and 'does not exist', causing them to reject legitimate
email.
Wietse
