I’ve been running Postfix on all my servers for many years now. Postfix has proven to be fast, stable, and reliable.
I’ve had some complaints, though. The main one is its filtering support. It doesn’t support filtering during the SMTP transaction. (Well, OK, recent versions do, but not very well.)
The other problem with filtering is that all of the plug-in filtering solutions stink. I am using Amavis, which is the best of them, but it still uses about 40MB of RAM for each new message, and is rather unstable.
So, I’m trying out Exim. It appears to be much more configurable than just about anything, though its queue manager is not as powerful as Postfix’s. From what I’ve read, Exim has the same peak throughput as Postfix, but a gentler ramp-up curve. That’s fine with me.
For me, the killer app for Exim is exiscan-acl. Spam and virus checking at SMTP conversation time, and a RAM overhead of about 1/80th of Amavis.
So far, I’m really liking it, though I’m having a small problem on my Alpha.