IPv6 sending
Delivering mail from an IPv6 address, which large receivers accept only from senders with valid reverse DNS and passing authentication.
What it means
SMTP works over IPv6 as over IPv4, and RFC 5321 covers MX hosts reachable over either. Receivers set stricter conditions for it. Google requires that the sending IP have a PTR record and rejects mail that misses its IPv6 rules with 550 5.7.1, citing PTR records and authentication. Microsoft rejects IPv6 senders without a reverse DNS record, and requires mail arriving over IPv6 to pass SPF or DKIM.
A sending server with IPv6 enabled may use it whenever the receiving MX host has an AAAA record, so a server that was never set up for IPv6 can start failing at such receivers. Either disable IPv6 for outbound SMTP or set it up fully: a PTR record for each sending address, a matching forward record, the range in SPF, and DKIM signing.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 5.2: IPv6 and MX Recordsrfc-editor.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes