bug#36937: 26.1; Sending mails doesn't set From: correctly

[Stefan Monnier <monnier@iro.umontreal.ca>]

>>> This problem started happening in builds from master around 4 days ago:
>>>
>>> - start emacs -Q
>>> - M-x mail (or any other form of sending email)
>>> - Set a From: value different from user@localhostname
>>> - Send mail using local sendmail
>>>
>>> The mail is sent with a from header seen by postfix (i don't know about
>>> other MTAs) as user@localhost, instead of the From: i set in the
>>> headers.  By user@localhost i mean the unix username running emacs and
>>> the hostname as returned by hostname(1).  It seems emacs is just
>>> ignoring the value of From:, or setting it in a way postfix doesn't
>>> understand.
>>>
>>> I know almost for sure it's an emacs problem because emacs 26 or an
>>> earlier version of emacs-snapshot is working without problems.
>>
>> Confirmed here, although it did not prevent me personally from sending
>> mail, since I had already set up postfix to rewrite such
>> user@localhostname addresses back.
>>
>> Reverting commit 3a59cc840693 (" * lisp/gnus/message.el: Reduce
>> redundancy with send-mail-function") fixed the problem.  It looks like
>> this commit changed some important options and looks NEWS-worthy.
>
> Most notably, it changed the default of message-sendmail-f-is-evil,
> which is now t.

Indeed, I aligned it with mail-specify-envelope-from.  
I guess I should revert this part.  But I'm still perplexed: `M-x mail`
shouldn't pay any attention to message-sendmail-f-is-evil when sending
the message.


        Stefan