@node Exim
@section Integration with Exim

This section is unaltered copy-paste of
@url{https://changelog.complete.org/archives/10165-asynchronous-email-exim-over-nncp-or-uucp, Asynchronous Email: Exim over NNCP (or UUCP)}
article by John Goerzen, with his permission.

@strong{Sending from Exim to a smarthost}

One common use for async email is from a satellite system: one that
doesn't receive mail, or have local mailboxes, but just needs to get
email out to the Internet. This is a common situation even for
conventionally-connected systems; in Exim speak, this is a "satellite
system that routes mail via a smarthost". That is, every outbound
message goes to a specific target, which then is responsible for
eventual delivery (over the Internet, LAN, whatever).

This is fairly simple in Exim.

We actually have two choices for how to do this: @command{bsmtp} or
@command{rmail} mode. bsmtp (batch SMTP) is the more modern way, and is
essentially a derivative of SMTP that explicitly can be queued
asynchronously. Basically it's a set of SMTP commands that can be saved
in a file. The alternative is @command{rmail} (which is just an alias
for sendmail these days), where the data is piped to
@command{rmail}/@command{sendmail} with the recipients given on the
command line. Both can work with Exim and NNCP, but because we're doing
shiny new things, we'll use @command{bsmtp}.

These instructions are loosely based on the
@url{https://people.debian.org/~jdg/bsmtp.html, Using outgoing BSMTP with Exim HOWTO}.
Some of these may assume Debianness in the configuration, but should be
easily enough extrapolated to other configs as well.

First, configure Exim to use satellite mode with minimal DNS lookups
(assuming that you may not have working DNS anyhow).

Then, in the Exim primary router section for smarthost
(@file{router/200_exim4-config_primary} in Debian split configurations),
just change @code{transport = remote_smtp_smarthost to transport = nncp}.

Now, define the NNCP transport. If you are on Debian, you might name this
@file{transports/40_exim4-config_local_nncp}:

@example
nncp:
  debug_print = "T: nncp transport for $local_part@@$domain"
  driver = pipe
  user = nncp
  batch_max = 100
  use_bsmtp
  command = /usr/local/nncp/bin/nncp-exec -noprogress -quiet hostname_goes_here rsmtp
.ifdef REMOTE_SMTP_HEADERS_REWRITE
  headers_rewrite = REMOTE_SMTP_HEADERS_REWRITE
.endif
.ifdef REMOTE_SMTP_RETURN_PATH
  return_path = REMOTE_SMTP_RETURN_PATH
.endif
@end example

This is pretty straightforward. We pipe to @command{nncp-exec}, run it
as the nncp user. @command{nncp-exec} sends it to a target node and runs
whatever that node has called @command{rsmtp} (the command to receive
bsmtp data). When the target node processes the request, it will run the
configured command and pipe the data in to it.

@strong{More complicated: Routing to various NNCP nodes}

Perhaps you would like to be able to send mail directly to various NNCP
nodes. There are a lot of ways to do that.

Fundamentally, you will need a setup similar to the UUCP example in
@url{https://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_manualroute_router.html,
Exim's manualroute manual}, which lets you define how to reach various
hosts via UUCP/NNCP. Perhaps you have a star topology (every NNCP node
exchanges email with a central hub). In the NNCP world, you have two
choices of how you do this. You could, at the Exim level, make the
central hub the smarthost for all the side nodes, and let it
redistribute mail. That would work, but requires decrypting messages at
the hub to let Exim process. The other alternative is to configure NNCP
to just send to the destinations via the central hub; that takes
advantage of onion routing and doesn't require any Exim processing at
the central hub at all.

@strong{Receiving mail from NNCP}

On the receiving side, first you need to configure NNCP to authorize the
execution of a mail program. In the section of your receiving host where
you set the permissions for the client, include something like this:

@example
exec: @{
    rsmtp: ["/usr/sbin/sendmail", "-bS"]
@}
@end example

The @option{-bS} option is what tells Exim to receive BSMTP on @code{stdin}.

Now, you need to tell Exim that nncp is a trusted user (able to set From
headers arbitrarily). Assuming you are running NNCP as the @code{nncp} user,
then add @code{MAIN_TRUSTED_USERS = nncp} to a file such as
@file{/etc/exim4/conf.d/main/01_exim4-config_local-nncp}. That's it!

Some hosts, of course, both send and receive mail via NNCP and will need
configurations for both.