IPv6 isn't SMTP

Thu Mar 27 02:08:33 UTC 2014

>To my knowledge, there are three impacts that IPv6 implementation makes on an SMTP implementation. One is that the OS
>interface to get the address of the next MUA or MTA needs to use getaddrinfo() instead of gethostbyname() (and would
>do well to observe RFC 6555’s considerations).

In practice it's considerably more complex than that due to MX
handling.  If you have multihomed hosts, or multiple MXes at the same
priority, you need to decide in what order to try them, and what to do
next if a connection attempt fails.  If one MX has an A record and
another has AAAA, do you always prefer the one with the AAAA?  If a
host has both an A and an AAAA, you probably try the AAAA first, so if
the AAAA connection fails, do you try the A or do you skip to the next
host?  The RFC is deliberately unhelpful here, and a fair amount of
fiddling is required to come up with heuristics that work well.

There are also some odd things in the spec.  For example, according to
RFC 5321 this is not a syntactically valid e-mail address:

mailbox@[IPv6:2001:12:34:56::78:ab:cd]

R's,
John