An A record is an MX record and is a missing MX....

Thu Apr 3 21:58:53 UTC 2003

Please forgive if this has been discussed, beaten, or decided previously 
here on the list.  A recent issue I encountered has prompted me to ask the 
following question.  What I am looking for is clarification regarding the 
"proper" way of implementing Mail Exchange records, etc. 

I have always been under the impression (or taught at least) that an MX 
record was necessary (required) for mail exchange.  I at least believed that 
this was the correct way.  Recently, we implemented a new mail server at our 
facility and started having some issues relaying mail to a few domains.  
Although this has already been resolved, I was under the impression that 
these two domains where the actual problem since I could not resolve an MX 
record for either one. 

Since then I have learned that some MTA's will look for an A record if it 
cannot find an MX record and use the A record instead. 

Is this acceptable (in a "best case scenario") as a correct method?  
Obviously some admins I have encountered are starting to host mailservers 
for sub-domains and domains without MX entries on their DNS zone records.  
Relying on the A record alone. 

Gerardo A. Gregory 