[PATCH 0 of 4] [RFC] sendbox rework

[agriffis@xxxxxxxxx]

This is an RFC complete reworking of my sendbox patch against
mercurial tip, based on conversation with Rocco the last time I posted
it.  In total, this patch has been previously posted three times:

2006-08-09 http://markmail.org/message/wvbraazolprwrvqx
2007-12-04 http://markmail.org/message/52k75aw2d7q4c74m
2008-08-13 http://markmail.org/message/lowdwylvs527v2mp

This rework ceases to bastardize mutt_write_fcc and attempts to create
roughly parallel interfaces for sending via smtp, sendmail and
sendbox.  A necessary difference arises because smtp and sendmail both
need a temporary file, whereas that would be wasteful with sendbox.
So the interfaces aren't perfectly parallel, but as close as I could
manage.

Additionally this rework does away with use_sendbox.  Instead the
preference order for sending mail becomes: sendbox -> smtp_url ->
sendmail.

Here is the original, unchanged description and rationale:

The purpose of this patch is to make it possible to send mail using
Courier IMAP's Outbox feature. In mutt, I avoided calling this
"outbox" because that term is already used in the mutt sources.
Besides, I think sendbox is more explicit.

The Outbox feature works like this: To send mail, it's possible to
nominate a mailbox that will send mails which are saved to it. This
mailbox acts like any other mailbox otherwise, including storing the
mails that are saved there.

This is desirable for numerous reasons, but here are the most
significant:

- on a slow connection, this allows the mail to be fcc'd and sent with
  the same transmission, rather than hitting the wire twice.

- sending mail from one's mailhost is nice because the mail originates
  from a good server, so your From address is valid and unquestioned

- single channel of mail transferral, easier network configuration and
  dealing with firewalls

- single point of authentication instead of two

- much easier to switch outgoing servers this way, instead of needing
  to reconfigure sendmail to use different outgoing servers

So this patch adds a setting "sendbox" which specifies a mailbox to
which sent messages should be saved rather than using smtp or
sendmail.  For example:

    set sendbox=imaps://my.server.com/INBOX/Outbox

Thanks, Aron