This new gem is from the SMTP Gmail FAQ at https://support.google.com/mail/answer/81126?hl=en
(Fun note: they call it the “Bulk Senders Guidelines”… hence apparently anyone running their own personal mail server falls in that category…)
“Additional guidelines for IPv6
The sending IP must have a PTR record (i.e., a reverse DNS of the sending IP) and it should match the IP obtained via the forward DNS resolution of the hostname specified in the PTR record. Otherwise, mail will be marked as spam or possibly rejected.
The sending domain should pass either SPF check or DKIM check. Otherwise, mail might be marked as spam.”