The Exim configuration follows this outline:
We accept mail for two classes of local domains. The ilrt is one; we accept email addressed to *@mail.ilrt.bris.ac.uk or *@ilrt.bris.ac.uk from the mailhubs at Bristol.
The other domains are those we host an MX for; we accept email addressed to recipients in those domains from anywhere.
In this latter case, we use per-domain alias files (with defaults) to direct email to local users.
Outgoing email is accepted only from local hosts (those on appropriate subnets) - although I might look at SMTP-AUTH or an IMAP-during-SMTP hack that's tied into the cyrus config at a later date.
For that outgoing email, if any of the envelope sender, From:, or Sender: are *@mail.ilrt.bris.ac.uk or *@ilrt.bris.ac.uk, we do a smarthost-style forward to the central Bristol mailhubs for address rewriting.
Otherwise, we handle the outgoing email delivery directly.
DIRECT_LOCAL = ilrt.bris.ac.uk, mail.ilrt.bristol.ac.uk and the ilk; OTHER_LOCAL = sosig.ac.uk, etc. (from /usr/local/exim/etc/other-local) RELAY_VIA_SMARTHOST = bris.ac.uk, bristol.ac.uk INCOMING_HUB = bristol hub(s) OUTGOING_SMARTHOST = Bristol hub(s) RELAYABLE_HOSTS = 34 subnet, localhost, etc. On mail arrival: if for DIRECT_LOCAL (and from INCOMING_HUB or RELAYABLE_HOSTS) deliver locally (generally via cyrus) - alias file is /usr/local/exim/etc/aliases (with optional forwardfile) if for DIRECT_LOCAL reject. # We only accept mail to this if it's been cleaned if for OTHER_LOCAL use otherdomain alias file /usr/local/exim/etc/aliases.domain (with *@domain default), then continue as per normal for local delivery otherwise, if not from RELAYABLE_HOSTS, reject. # Mail from RELAYABLE_HOSTS going outwards: If any of: Envelope sender, From:, Sender: are in DIRECT_LOCAL or RELAY_VIA_SMARTHOST then relay through OUTGOING_SMARTHOST # We check all mail with bristol addresses for validity via the central hubs Otherwise, send directly.