Staver asked about the issue when replying, not original compose of e-mail.
Originally Posted by staver
I can't change the "sender" if I'm replying to an email.
To REPLY to a message, you can't change the SEND USING. If you must REPLY via different account, choose FORWARD from the open message and put your address in there, and change the SEND USING.
Hope that clears it up.