Who should set the Sender: header field?

Hi all,

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 first?


Best
Ale
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