mail coming without MX; how ?

[Andrew McNamara]

>> Most Mail Transport Agents will fall back to the A record if no MX
>> records are found. This precident was set by sendmail, and woe betide
>> any implementation that ignores precident, but it would be foolish to
>> count on all MTAs behaving this way.
>
>With SMTP to you _can_ count on this behaviour. Quoting RFC 2821:
>
>  5. Address Resolution and Mail Handling
>  [...]
>  The lookup first attempts to locate an MX
>  record associated with the name. [...] If
>  no MX records are found, but an A RR is
>  found, the A RR is treated as if it was
>  associated with an implicit MX RR, with a
>  preference of 0, pointing to that host.

RFC 2821 is relatively new (certainly newer than most MTAs), and while
the popular ones have made some effort to comply with it, many others
still struggle to comply with RFC 821.

-- 
Andrew McNamara, Senior Developer, Object Craft
http://www.object-craft.com.au/