[sympleko]

Using the drop-down to change the "from" field selects the account you send from, and likely the server you send through. You just want replies to be directed somewhere else. That's what "Reply-To:" is for.

To set this for individual messages, do this when composing (copied from Mail help):

1. In the New Message window, click the Customize pop-up menu in the lower-left corner of the header area, and then choose Customize.
The header area expands to show all the items you can include when addressing the message.
2. Click the checkbox for each item you want to include:

* Reply To: Indicates the email address where you want to receive replies.
3. Click OK.

Mail user agents SHOULD respect the reply-to header and automatically send replies in that direction. See RFC 822. But the point is this could be any address you want, not necessarily one for which you have a mailbox. It doesn't even have to be a valid address--which of course would be bad because you wouldn't get any replies. :-)