I'm receiving mail addressed to some 30 aliases in the same account. As most of these addresses are task oriented, I filter them to separate folders.
For those folders I've set the identity (somewhere in the folder properties) to the matching address. Now when I send a mail from one of those folders, the From: address as defined for that folder is being used. This method suits me best.
Another option would be to use a macro like this in your reply template:
%From="%OTo"%-
This will place the old To: header in the From: header of the new message. Note that this will cause you problems as soon as you're replying to a message that has multiple recipients in the T: header, where you were mentioned as CC: or BCC: and finally when you're replying to messages from a mailing list.
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