Application Development Software

Version is 6.3 on solaris 9
The system was installed with the help of outside vendor.
After installation, it appears that the messaging server sees email addresses
for our own domain, e.g. userxxx@my.top.domain as mails for the
user's account name "userxxx" on this same solaris 9, instead of
looking up the MX record "my.top.domain" on our dns server to send the mail
In other words, it seems to identify itself as "my.top.domain" instead of
its actual fqdn "solaris9-machine.my.top.domain"
We have used sendmail for ages, and sendmail's default config. knows about
mx record of our own domain.

My workaround is to create mail aliases with 2 types:
1. for userxxx whose mail box is on this solaris 9
userxxx@solaris9-machine.my.top.domain: userxxx@my.top.domain

2. for useryyy whose mailbox is on another machine
useryyy@my.top.domain: useryyy@otherhost.my.top.domain

But this is not a neat way to do things.
Could someone tell me how to make the JES Messaging Server
knows how to resolve addresses @my.top.domain using the DNS mx record?