Help with multiple domains

[Kelly Sauke]

I'm trying to set up my box so that I can recieve mail for multiple
domains on one box.   Both domains will resolve to the same IP address.
The problem I'm having is when  a message is sent to the secondary
domain I get this error in the email bounced back to the sender.

553 secondary.domain.com. config error: mail loops back to me (MX
problem?)
554 <username at secondary.domain.com>... Local configuration error

I can recieve mail for my primary domain just fine.  There are no
problems and because of that I don't believe its a sendmail problem.  It
might be however..I'm still new to sendmail.  Is there something that I
should be doing in the db.secondary.domain file for the MX record.
Right now it is


secondary.domain.com    IN    MX 30    hostname.primary.domain.com

Thanks for your help.

Kelly Sauke