multi smtp server support

[christophe barbe <christophe barbe ml online fr>]

I'm looking at the code in libbalsa/send.c to add support for multi smtp
server.

To send a message we call  
	347: libbalsa_message_send()
This function queue the mail in the outbox with the call to
	282: libbalsa_message_queue(message, outbox, fccbox, encoding)
and then proceed the queue with
	432: libbalsa_process_queue(outbox, encoding, smtp_server,
smtp_authctx)
Then all mails in the outbox are send.

Note that smtp parameters are not passed to the message_queue fonction but
to the process_queue one.

(I've noted that the mails are added in the sentbox when they are queued
not when they are sent. I believe this is a mistake.) 

My question is would it be better to : 

1/ have one outbox by smtp server even if we hide the outbox and only show
a unified one. The problem with this solution is to show a unified box.

2/ use an internal HEADER field (par example: BALSA_INTERNAL_ID) to store
the ID associated with the message and then use this field to select the
smtp server.
This header field should be remove before sending (then we need a new
function in libesmtp).

3/ use explicitly an outbox by smtp server. Then when you had a server you
are asked to specify an outbox and this box is seen in your mailbox tree.

Christophe


-- 
Christophe Barbé <christophe.barbe@online.fr>
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