MX records

[Ted Wood]

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