Do I really need an MX record? (for e-mail to work)

[John Coutts]

In article <drr06s$1fta$1 at sf1.isc.org>, csmith.lunchmeat at dyndns.org says...
>
>No, you do not need to have an MX record in order for e-mail to work.
>
>For a domain, example.com, if there is an A record for that host and the 
>machine at the IP address which the A record points to is configured to 
>accept mail for the domain, then no MX record is required.
>
>I know of not a single commonly used MTA which will fail to properly 
>deliver mail in such a scenario, and any MTA which does fail in such a 
>scenario would be in serious violation of numerous RFCs.
>
>Nor do I know of any MTA which will refuse to accept mail from a domain 
>which has no MX record but which does have an A record. Certainly some 
>administrators may have configured their mail server in some fashion so 
>as to reject mail from such a domain, but we can't do much to prevent 
>that.
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In theory you are correct. In the fight against spam however, many mail servers 
do fail on an "A" record check (for example my own mail provider at pobox.com). 
There are some that even go so far as to reject mail if the sending domain is 
not prepared to accept responding email by advertising an "MX" record. It may 
not be accordong to RFCs, but it is a fact of life in todays email system.
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>As regards MTAs which refuse to accept mail from mail servers located on 
>DSL networks, I think you mean they refuse to accept mail from servers 
>whose IP addresses are listed in one of several lists of "dynamic" or 
>DUL addresses. Not all IP addresses on DSL networks are dynamic. Many 
>DSL providers provide static IP address assignments and make sure their 
>IP addresses are not listed in such lists.
>
>And, even if he is receiving mail at a server on a dynamic IP address 
>does not mean his outgoing mail is delivered from the same server. He 
>may use an ISP provided SMTP server to send outgoing mail.
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Again, theory and practice diverge here. I don't necessarily agree with the 
practice, but some mail servers do a reverse lookup on the sending IP, and if 
it contains "DSL" (or other commonly used conventions) in the name, it is 
rejected. It is not a common practice, but it is being done.

One practice that I completely disagree with is that of rejecting mail if the 
"A" record does not match the "PTR" record. Checking for the existence of a 
"PTR" record is a questionable practice, but requiring it to match the "A" 
record is pushing things a bit too far. The domain owner does not necessarily 
have control over his/her own "PTR" record, and ISPs in general do not do a 
very good job of maintaining these records.

J.A. Coutts
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>-- 
>Christian Smith
>Dynamic Network Services, Inc.
>
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