Hi,

I'm trying to make sending mail over SMTP asynchronous.  For this, I'm
working on 'smtpmail-send-it'… but I think that that function does too
much and is hard to reason about.

This patch just splits 'smtpmail-send-it' work into three functions:
prepare a mail and queue it or send it.  There should be no change in
functionality (it is mostly code refactoring).  I've tested it a bit
with sending and queueing and this seems to work correctly but this
needs way more eyes and tests.

BTW, does anyone know the purpose of 'errbuf': I see it is created then
erased and then killed?