This patch adds support for TLS-wrapped submission connections.
Instead of clients establishing a connection over plain text and then
using STARTTLS to switch over a TLS connection, this new mode allows the
clients to connect directly over TLS, like it's done in HTTPS.
This is not an official standard yet, but it's reasonably common in
practice, and provides some advantages over the traditional submission
port.
The default port is 465, commonly used for this; chasquid defaults to
systemd file descriptor passing as for the other protocols (for now).
glog works fine and has great features, but it does not play along well
with systemd or standard log rotators (as it does the rotation itself).
So this patch replaces glog with a new logging module "log", which by
default logs to stderr, in a systemd-friendly manner.
Logging to files or syslog is still supported.
The default INFO logs are more oriented towards debugging and can be
a bit too verbose when looking for high-level information.
This patch introduces a new "maillog" package, used to log messages of
particular relevance to mail transmission at a higher level.
If there's an alias to forward email to a non-local domain, using the original
From is problematic, as we may not be an authorized sender for it.
Some MTAs (like Exim) will do it anyway, others (like gmail) will construct a
special address based on the original address.
This patch implements the latter approach, which is safer and allows the
receiver to properly enforce SPF.
We construct a (hopefully) reasonable From based on the local user, and
embedding the original From (but transformed for IDNA, as the receiver may not
support SMTPUTF8).
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.
