I like it when email clients use the "To" field not the "From" field when you Reply to an email in your Sent Mail.
I often think of a follow-on to an email I've just sent, and the above behaviour makes Reply perfect for that. Gmail gets this right. Outlook Web App (Office 365) does not.
I guess if a client was really clever it could change the label "Reply" to "Follow On" or something when on an email you wrote.