how to configure postfix to send local mail to another local server?

Question

I have a pair of mail servers that are comprised mainly of Postfix, Procmail and Cyrus-IMAP and they both work fine. The IMAP servers are configured in a simple master/replica pair. All 'normal' mail is accepted by the master's Postfix which processes it through Procmail for delivery to the Cyrus-IMAP running on the same server. Both servers are capable of running like this but only the master actually receives external email.

At present, local accounts on either box send mail (for example, generated by cron jobs) through its local Postfix which is delivered locally to the Cyrus-IMAP on the same box.

However, Cyrus's replication doesn't like changes happening directly on the replica so I would prefer for the replica to deliver its local mail to be delivered to the master.

I see two ways of achieving this:

• change the replica's local transport to deliver mail to Postfix on the master server

or

• change the replica's procmail to deliver mail to Cyrus-IMAP on the master server.

There is a third way which is for the replica server to relay everything to the master server (i.e. both internal and external emails) but that seems somewhat unnecessary. I don't need to change how external mail is handled - it's ok for either box to send external email via its local Postfix which then relays it externally.

What I really would like to do is just change how internal emails are processed on the replica so that it delivers them to the master, either into Postfix or into the IMAP server there.

Existing, working, configuration of my local transport in postfix's master.cf:

procmail-local  unix  -       n       n       -       -       pipe
    flags=OR user=cyrus argv=/usr/bin/procmail -t -m RECIPIENT=${mailbox}
    SENDER=${sender} EXTENSION=${extension} /etc/procmailrc

Existing, working, configuration of delivery to Cyrus in procmailrc:

DELIVERMAIL="/usr/lib/cyrus/bin/deliver -r $SENDER -a $RECIPIENT $RECIPIENT

Solution

I'd do it earlier still - by removing your domain name from mydestination, mail for that domain should be delivered to the server listed as MX for your domain. If you want to specify a server, as opposed to using the MX, you should use the file /path/to/postfix/etc/transport for this. That file should then look like this:

# all mails to our own domain should go to our main incoming server
ourdomain.com    smtp:[mainserver.ourdomain.com]

Please note that the [] marks are important - without them, your server will still lookup the MX record for mainserver.ourdomain.com.

After editing the file, use the command postmap /path/to/postfix/etc/transport

You also need to make sure that postfix knows to use that file, by editing /path/to/postfix/etc/main.cf with the following line:

transport_maps = hash:/path/to/postfix/etc/transport

if your system uses db files. If instead it uses dbm files, the line should be

transport_maps = dbm:/path/to/postfix/etc/transport

Comments

[starfry]

I had been playing with transport maps before posting the question but without success. Seeing your suggestion, I tried what you described but it didn't work for me. I have pasted my main.cf at pastebin.com/UP5pJWsK. I created the transport file using the IP address of the master server (as in localdomain smtp:10.0.2.2). I did postmap and postfix reload. I also tried a stop/start cycle).

[starfry]

My domain's MX is actually at an external service provider and my master uses fetchmail to grab mails from there. While I can see the sense in this it wouldn't work in my case (even if I wanted to relay the mail externally, am unable to set up MX records for a subdomain like myhost.mydomain.com).

[starfry]

Well I've been playing around a little more and I have got it to work by placing .mymaildomain.co.uk smtp:10.0.2.2 in the transport file (I added the leading dot so it maps subdomains used the actual domain name rather than localdomain)

[Jenny D]

OK, I'll amend my answer to match that.

[starfry]

When I put .mydomain.co.uk smtp:[master.mydomain.co.uk] in the transport file it does not work. When I put .mydomain.co.uk smtp:10.0.2.2 it works as expected. I needed the leading period because the sending server is a subdomain. I have master.mydomain.co.uk defined and I can ping it so I know it resolves. I have not fogotten postmap and I have also restarted postfix, just in case. Any ideas why it's failing to map using the name?

[Jenny D]

Do your logs give any hint?

[starfry]

Well I did check my logs and I have been investigating what I found in there (Host or domain name not found. Name service error for name=master.mydomain.co.uk type=AAAA: Host not found). First, I wondered why it was trying ipv6 lookups so I added inet_protocols = ipv4 to main.cf but all that did was change the error message from AAAA to A. I then tried something else and it now works: I added a record for master.mydomain.co.uk to an internal DNS server. So this begs the question of why the /etc/hosts lookup does not work but we're probably going beyond the scope of my original question!

[starfry]

I have gotten to the bottom of the name resolution issue. How Postfix resolves smtp hostnames is defined by the main.cf smtp_host_lookup and this defaults to ussing dns only. By explicitly configuring this as smtp_host_lookup = dns,native, Postfix will use native mechanisms (as defined by /etc/nsswitch.conf, typically so that /etc/hosts is used). So - problem solved :)

[Alexis Wilke]

Are you sure that relay_domain = domain.com is safe? On my end I use mynetwork = IP, IP, IP... which seems way more secure (although you have to make sure you keep your IPs up to date, of course.)