+--------------- | Barry Margolin wrote: | > Mail servers don't try to figure out what you mean. They just take the | > name after the "@", and look up the MX record to see where mail should be | > delivered (if there's no MX record they'll look for an A record). | ........ | Are you absolutely sure that MX records are not required? | My understanding (and experience) is that they are mandatory, | and the RFC seems to say so: | |
Well, RFC 974 was obsoleted by RFC 2821, and now has "Status: HISTORIC". RFC 2821 [the "rewrite" of RFC 821] quite explicitly agrees with what Barry said above:
Simple Mail Transfer Protocol Obsoletes: 821, 974, 1869 April 2001 Updates: 1123 ... 5. Address Resolution and Mail Handling ... a DNS lookup MUST be performed to resolve the domain name. ... The lookup first attempts to locate an MX record associated with the name. If a CNAME record is found instead, the resulting name is processed as if it were the initial 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.
Perhaps you were thinking of the next few sentences, which *don't* contradict the previous but do require failure if an explicit MX RR is present but unusable [regardless of any A RR]?
If one or more MX RRs are found for a given name, SMTP systems MUST NOT utilize any A RRs associated with that name unless they are located using the MX RRs; the "implicit MX" rule above applies only if there are no MX records present. If MX records are present, but none of them are usable, this situation MUST be reported as an error.
In any case, it is certainly the case that an A RR *without* an MX RR for the same domain is quite legal, and is supported by every major piece of mail relay software I know of. I have administered numerous machines which lacked MX RRs, and they had no trouble at all receiving mail.
----- Rob Warnock
627 26th Avenue San Mateo, CA 94403 (650)572-2607