Currently Being ModeratedOct 19, 2012 8:54 PM (in response to Lisa Hodgen)
Mail > Preferences > Composing > Send new messages from: Account of selected mailbox
Currently Being ModeratedOct 19, 2012 9:11 PM (in response to Lisa Hodgen)
There is a long thread on this issue: Re: Mail bug in Mountain Lion
If you leave the email in the inbox, it will reply to the address that it was sent from.
Otherwise, it replies to the top listed email address in the mailbox account list.