changing a qmail domain

[KayakRob]

I am new to qmail (I've never heard of it until I was tasked to resolve this issue.

We send messages to a mailinglist.

The messages are received as

from :admin@abc.com
to :winnums@mailinglist.abc

(message...)

Many ISP's block the email because .abc is not a valid domain name.

How do I change it so the messages will look like

from :admin@abc.com
to :winnums@mailinglist.abc.com

(message...)


So basically I want to change the domain name.