change to IP's master domain not changing everywhere

Hi!

I installed Virtualmin using one domain name, but then changed the master domain to another one. It seems to have remapped the new master domain correctly in most places, but sometimes the original domain crops up. I'm working on finding and fixing these instances, but one that I'm working on now has me stumped.

When I look at all the headers in email sent from my server I get this:

Received from NewDomain.com (OldDomain.com [1.1.1.1]) by....

How do I change the value "OldDomain.com" to "NewDomain.com" as this is incorrect.

Thank you!