Usermin is mis-configured on your box. I'm guessing it was installed before the installer got this right.
The instructions for fixing it are in the troubleshooting docs here:
http://www.virtualmin.com/documentation/id,troubleshooting_common_problems/
#usermin_webmail_sends_with_incorrect_fromaddressBTW-This is a client-side issue, so if you have similar problems with other mail clients, the right solution is usually in the client. Postfix can be configured to perform canonical map translation on these email addresses, if the client isn't smart enough to set the From: address correctly automatically. I seem to recall we added an option in Virtualmin to auto-generate these maps, but I'm not seeing it at the moment.