Re; MX records

[Pete Peterson]

> From: Ted Wood <twood at twu.edu>
> To: "Bind-Users (E-mail)" <bind-users at isc.org>
> Subject: MX records
> Date: Fri, 15 Mar 2002 14:10:38 -0600
> 
> 
> I've got a question about MX records.  My domain, twu.edu, has a mail server
> which has an A record which points to just the domain name like so:
> @	IN	A 	205.165.49.231
> It is also the preferred mail server:
> @	IN	MX	0 venus.twu.edu
> And of course it also has its own A record:
> venus	IN	A	205.165.49.232
> 
> There are also some other mail servers with lower preference values:
> @	IN	MX	10 exchange.twu.edu
> @	IN	MX	20 vulcan.twu.edu
> etc.
> 
> Now, as I understand MX records, mail sent to somebody at twu.edu should try
> venus first, then exchange, then vulcan.  However, if somebody at twu.edu has
> their account on exchange or vulcan, the mail never gets there.  If I send
> it to somebody at exchange.twu.edu or somebody at vulcan.twu.edu it gets there.
> Why isn't this working?  Is it because of this record: @	IN	A
> 205.165.49.231?  It seems to be that since twu.edu points to a particular
> IP, mailers are just looking it up directly and not even looking for the MX
> records.  Thanks in advance for any help.
> Ted Wood

You're misunderstanding how MX records work.  All servers specified in 
a domain's MX records are expected to be able to deliver to
anyuser at thatdomain.com as well as to users at other domains which may
specify that machine in an MX record.

If you specify several machines in MX records, the sender will try them in
numerical priority order until it finds one that *RESPONDS*.  

If there's no response after trying all the machines, a transient failure
occurs and the sender must retry later.

If there *IS* a response, **EACH** of the machines is expected to be able
to accept the mail and deliver it to the recipient, either directly or
through store and forward to some other mail server.  If a server
*RESPONDS*, but doesn't know the recipient, the sender does *NOT* try the
other servers.

There's no concept of trying a second machine because the first one is
misconfigured (missing users) --- only for trying machines in succession
when the previous machines DON'T RESPOND.

     pete

-- 
        pete peterson
        Teradyne, Inc.
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