more on mail queue lossage
I think the simplest way of dealing with messages in flight across
mail transport agent changes would be to define a simple queue
structure to hold the messages across mta installations.
Probably the simplest mechanism is a directory with one file
per message, first line being envelope from address, subsequent
line(s) being destination address(es), then a blank line, then
the message (headers and all).
Policy should be that a mail transport agent postrm should move any
undelivered mail messages into this queue with this format and that the
postinst should move messages from this queue to the new mta's queue.
ssmtp would require special effort (either refuse to install using a
check in preinst or a queue run job which checks the error code from
/usr/sbin/sendmail). An mta shouldn't be considered installed until
all messages have been succefully moved out of this temporary queue
(technically, it's ok if unreasonable messages get bounced, and this
should be considered a success).
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