I’ve been using Postfix for around 5 years now and it’s been a great solution for mail. Initially I used a single instance, but as the mail volume grew it started to cause bottlenecks and frustrations. The solution was to move from one instance to 3!

Basic Flow

The theory is to have each instance of postfix performing a specific role and using the standard communication mechanism between them. Each instance operates independently and using it’s own queues while sharing a common log.

Submission Instance

The submission instance listens only on localhost and has few additional checks.
This does open a potential issue if the host is compromised as it will be able to access the port directly, but if compromised then there are other issues 🙂

Input Instance

My input instance listens on the usual SMTP and SMTPS ports for all available interfaces, allowing local access as well as external. It has a lot of additional checks, spam checking, DMARC and so on as well as rate limiting.

Output Instance

All mail that is accepted by either of the input instances arrives here, either for local or external delivery. This is the only instance that delivers mail and as such only the appropriate checks are carried out.

Installation

The Postfix website has a page explaining how to set up the various instances and it’s a great place to start, but in my experience it takes a while to get the configuration working exactly as you want it. Adding additional checks sometimes has unexpected consequences, so the usual guide of “make small changes, let the dust settle” applies.
As postfix makes use of chroot after the initial installation I found a number of files that were required weren’t available, so I had to copy them across into the correct places within the directory structure created. This has meant that following updates some files and libraries are out of date and so had to be updated manually. Log entries are made for such files following a restart.

Start/Stop/Reload

This doesn’t change and acts on all instances.

$ sudo postfix reload postfix/postfix-script: refreshing the Postfix mail system postfix-out/postfix-script: refreshing the Postfix mail system postfix-in/postfix-script: refreshing the Postfix mail system

Queue Checking

After making the changes it took me a while to always remember how to check the queues! It’s explained in the explanation but the fact the command still works and doesn’t tell you can be a bit unsettling.

$ mailq Mail queue is empty

Checking all the instances requires the use of postmulti.

$ postmulti -x mailq Mail queue is empty Mail queue is empty Mail queue is empty

Each instance reports separately, hence the 3 lines in the response.

Summary

While it took a while to get every instance running as I wanted, the advantages of having each instance running at their own speed has been a huge increase in throughput with a reduction in the load on the machine.