this is quite easy, I searched for it last month
Normally on your screen you should have 2 links in your left upper corner,
click on Module Config and go to From address options
This is the right place to change From: address to use when sending email manually
and other related details.
You can also set Domain to use in From: address which could be useful
and also another interesting option:
Include browser IP in X-Originating-IP header?
if you click no you browser IP will not be shown on the other end in the message source.
The above is valid for Webmin with admin access, I'm not 100% sure about Usermin available options.
Hope this helps