I'm currently running Exim4 on Debian as my primary MTA, which sits in front of my windows email server (they are on the same lan). Mail comes into the MTA which is the main (lowest) MX record for the domain, it processes the mail, does a few spam checking routines and then if it all checks it out opens an SMTP connection to the windows box and delivers the mail. My problem is the load on my MTA (Debian) is so high that I need to look into a second local MTA to alleviate some of the traffic.

Would it be possible to build another near identical Debian box, with the same exim4 config set to pass mail on the the windows box (again on the same LAN), and assign it an equally low MX record as the primary MTA? Am I correct in assuming it would "randomly" choose which MX to deliver to?

I don't need anything complex like load balancing I just want approximately half of the mail to reach each of the MTAs and can't think of a simple way to do it.

Thanks for any insight anyone can provide.

Yes you can do that though it's not true load balancing, it will provide you with a rough split of traffic. It will also depend on your top talkers and which server they have resolved and cached.

Are you handling spam/av at SMTP time, i.e. not accepting the email if SpamAssassin thinks its spam?

How are you handling load balancing your traffic outbound?

I've decided to go with round robin DNS instead of equal MX records. That way both incoming and outgoing email get balanced across both MTAs. Seems to be working well so far. For spam the debian MTAs do clamav, greylisting, general rfc compliance checks, dns-bl, and blacklisting of many large foreign CIDR blocks. Anything that gets past that gets sent on to the windows server which is running spamassassin, kaspersky av and various content filters. I have recently removed our greylisting as I couldn't find a proper way to share the triplet cache across both mtas and in general it seems to have caused us more grief than benefit since we started using it a few years ago.