UNIX hosts and MX "best practice"
Jim Reid
jim at rfc1035.com
Sat Jan 29 09:25:50 UTC 2000
>>>>> "Barry" == Barry Margolin <barmar at bbnplanet.com> writes:
Barry> In article <23058.949106728 at gromit.rfc1035.com>, Jim Reid
Barry> <jim at rfc1035.com> wrote:
>> A bigger issue is the impact on the mail delivery and queues
>> when mail is sent to hostnames that don't have MX
>> records. Suppose that mail is sent to user at foo.example.com and
>> there's no MX for foo.example.com. The A record is looked up
>> and the mail system will try to connect to port 25 on
>> foo.example.com and speak SMTP. If foo.example.com doesn't have
>> an SMTP listener, that connection will not succeed. The mail
>> will get queued and sendmail (or whatever) will keep trying to
>> deliver it for a few days or so before giving up. The mail
>> system can't tell the difference between a remote system that's
>> temporarily switched off its SMTP listener and one that *never*
>> has such a thing. So the mail system's queues get cluttered
>> with undeliverable mail and the postmaster gets a
>> headache. Meanwhile the user is unhappy because their mail
>> hasn't been received. Maybe the user was wrong to send mail to
>> foo.example.com when it didn't have an SMTP listener. But if
>> this host had an MX record, its mail could be delivered
>> somewhere, even if it was only to some mail system that bounced
>> the message because the email address was wrong.
Barry> I assumed we were only discussing whether a host should
Barry> have an MX record pointing to itself, versus just depending
Barry> on the A record.
IIRC, the original question was "Should my hosts have MX records or not?
What's the best practice?"
Barry> If its mail is supposed to be delivered
Barry> somewhere else, it's intuitively obvious that it needs an
Barry> MX record to get it there. Why would you have people send
Barry> mail to user at foo.example.com if it doesn't run a mail
Barry> server and there's no MX record redirecting its mail?
Because users and software can do stupid things, like confuse a
hostname with an email address.
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