I've got virtual min setup with 2 domains, let's say domain1.com and domain2.com along with their dns entries etc.
I've setup 2 email accounts, firstname.lastname@example.org
thourgh virtual min with usernames admin.domain1 and admin.domain2.
When I'm logged in into the webmail (squirrelmail or any other imap software) with either admin.domain1 or admin.domain2 and try to sent an email it is always changes the from field to admin.domain1@hostname or admin.domain2@hostname rather the actual domain.
Hostname is something like somename.myispname.com
Note that these domains point to a different IP as these are setup for testing prior to migration. However, I have setup Bind to listen on local dns (/etc/resolv.conf have 127.0.0.1 as 1st entry) and is resolving fine inside the machine.
Sendmail is configured manually and not via webmin or anything like that.