bug#14487: [Wish] Should ask before customizing send-mail-function

[Andrey Paramonov]

Hello Emacs developers!

When sending e-mail message for the first time, Emacs 24 asks for send 
method to be used. This is very handy, as in previous versions Emacs 
would use system mail transport, which is often not configured. Emacs 24 
now says:

Emacs is about to send an email message, but it has not been
 configured for sending email.  To tell Emacs how to send email:

 - Type `mail client' to start your default email client and
   pass it the message text.

 - Type `transport' to invoke the system's mail transport agent
   (the `e:/acd/utils/sendmail.exe' program).

 - Type `smtp' to send mail directly to an "outgoing mail" server.
   (Emacs may prompt you for SMTP settings).

I selected `mail client', but unfortunately my mail client GMail reported:

Oops!
Message too long.

(or something like that). It means I cannot use my mail client to send 
mail reliably, and I'd like to revert my choice. But now it's not clear 
how to do that because message-mode wouldn't ask me again.

So I propose:

1) sendmail-query-once should use your choice to send message this time.
2) After message is sent, sendmail-query-once should ask user whether 
she wishes to save the choice.

Best wishes,
Andrey Paramonov

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