When we permanently failed to deliver to one or more recipients, send delivery
status notifications back to the sender.
To do this, we need to extend a couple of internal structures, to keep track
of the original destinations (so we can include them in the message, for
reference), and the hostname we're identifying ourselves as (this is arguable
but we're going with it for now, may change later).
This patch tidies up the Procmail courier:
- Move the configuration options to the courier instance, instead of using
global variables.
- Implement more useful string replacement options.
- Use exec.CommandContext for running the command with a timeout.
As a consequence of the first item, the queue now takes the couriers via its
constructor.
This patch makes the queue read and write items to disk.
It uses protobuf for serialization. We serialize to text format to make
manual troubleshooting easier, as the performance difference is not very
relevant for us.
