Expanded from is not what I am expecting


I have two servers that I am using right now.

"web" server has its sendmail version 8.14.1 running. It has its dns
pointing to the "dns" server. The hostname of "web" is
web.something.com. The hostname of "dns" is dns.something.com.

When I am using mutt to send an email from "web", I am getting the
error of "Error sending message, child exited 67 (User unknown.)".
From what I can see, the email is coming back with this error message:
The following addresses had permanent fatal errors:
(reason: 550 5.1.1 <user@xxxxxxxxxxxxxxxxx>... User unknown)
(expanded from: user@xxxxxxxxxxxxx)

I am trying to figure out what and why the email address is being
expanded from user@xxxxxxxxxxxxx to user@xxxxxxxxxxxxxxxxxx

Can anyone give me a starting place...

I will be glad to give any more info that is needed!