using Thunderbird, for one, it is easy to modify the From: field of the message
being edited, possibly setting multiple mailboxes. However, that software
omits to add a Sender: field, which is required in such cases. Where's the
bug? Options are:
* The MUA should prompt for a Sender:, or
* the submission server adds Sender according to the logged-in identity.
I realize that both might take care of that, but where should I file a bug
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