Eli Zaretskii replied with a concern:
>> +;; It's the default mail mode, so it seems OK to use its features.
>> +(autoload 'message-bogus-recipient-p "message")
>> +(autoload 'message-make-address "message")
>> +(defvar message-send-mail-function)
>> +(defvar message-sendmail-envelope-from)
>
> What if the user overrides that default, and uses a different mail
> mode?

To which I replied:
> Then I guess that user will have some unused functions loaded, as well
> as some variables defined that she has no use for?  I was basically just
> cargo-culting the existing code.
>
> Existing code in emacsbug.el has 
>
>   ;; It's the default mail mode, so it seems OK to use its features.
>   (autoload 'message-bogus-recipient-p "message")
>   (defvar message-send-mail-function)
>
> I just added one more function and one more variable, and moved the
> whole bunch, including comment, earlier in the file, because I used
> those functions earlier in the file and thought it logical to place the
> definitions before the uses.
>
> Autoload of one function causes the entire file to be loaded, does it
> not?  So my additions are more for documentation purposes than any
> actual functional change.

Does anyone have any further thoughts?  I consider existing behaviour a
bug, and I believe my patch fixes it in a relatively unobtrusive manner.
message.el is already autoloaded today, and my patch does not change
behaviour if the user has already told message.el how to determine
envelope-from, or if the guessed address would be sane.