max limit in IMAP server processes

The cyrus imap server support a maximum of 300 sessions on Mac OS X Server, meaning when your clients use Mail.app, this limits the total user count to just above 60. Mail.app is a session eater. It likes to open 4 sessions per account, and I see users with even more now and then. Oh, and when you get close to 300 sessions, Open Directory likes to get a headache, making your whole server become unresponsive..

So Mac OS X Server unlimited supports a maximum of 60 mail users 😉

(Yes, I'm frustrated with that indeed.)

So Mac OS X Server unlimited supports a maximum of 60 mail users

I have to admit I haven't encountered this - I'm running 250+ users on a single mail server and haven't run into any problems (although, I concur, I haven't looked at who's using IMAP vs. POP).

Even if there is a limit, you can use mail clustering in Leopard to spread your load over multiple mail servers.

Typically there shouldn't be any problems up to 300 users.
Above that, it will depend on many parameters (bandwidth, usage patterns, mail clients, etc.) whether you will need to throw more servers at it. I have seen servers running with 700 users, while others max out at 300.

Assuming you only run mail services, pretty much any XServe will be able to handle 30 users just fine.
You won't need large amounts of RAM either.
I'd look for the "smallest" XServe with 4GB RAM or more and rather invest money in a good raid setup.

Of course, if you want to add additional services to this server in the future, you may need a different setup.

Our server started having trouble anytime the VM Size would get over 320GB. It could happen with just over 30 IMAP connections but sometimes it would be over 50 before we saw the server start to struggle. We would get the Open Directory problem as well and the server was not able to function if the VM Size got over 370 GB. We worked with Apple for a couple of months to get data captures and we could reproduce the problem for them anytime, but they did not seem interested to find a solution. Trouble ticket is still open and no response from Apple for a month now.

Yes, absolutely fine.

We have 215 configured mail account on our server. All of them connect using Apple's Mail program via IMAP. We use both squirrelmail and @mail (running on a 2nd xServe) as web interfaces to the mail server for off site access. Somewhere around 40 of the users also use iPhones to access mail via IMAP. This equates to around 650 concurrent IMAP connections during the day.

Our total mail store is only around 80GB. Its kept on a 7 drive xServe RAID (Major overkill). The mail server is an 8 core Intel xServe. 1GB Quota limit / 10MB attachment limit.

BTW: A full backup can take up to 5 hours. And to do it right the server needs to be off. 😟

We experience major slow downs 2 to 3 days a week. During all of these slowdowns the hardware shows no indication of heavy load. We have had Apple VARs verify the system is configured properly, spent hours on the phone with AppleCare. Its all correct according to them. Apple did finally admit there are performance issues and that the issue exists in both the Tiger and Leopard servers. Their new lit on Snow Leopard claims support for thousands of concurrent connections so we hope they have/will resolved these problems and their server will become a good solution for business. Maybe with ZFS we will get live backups too.

Some things to avoid as we have found they can cause performance problems. Limit your use of the Server Admin tool, specifically the Mail Maintenance tab, never leave it open. Encourage good daily mail management. Once your mail store grows to a larger level and mail accounts start to fill, people will start moving GB's worth of mail around in large blocks, the server will likely slow to a crawl for hours if too many people do this at once.

Use Mailbfr instead of the reconstruct options in Server Admin.

I greatly respect pterobyte's knowledge in this area, but from my experience, using the best Apple hardware we could by at the time. I feel more than 150 IMAP users will likely run into performance issues at one time or another using Apple's mail server. But until you do, it will seem blazingly fast. It just hits a wall and starts to crawl from time to time.

With all due respect, that many concurrent connections will bring any single mailserver to a crawl,
and underscores the need for distributing services (ie: distribute accounts over multiple servers when getting to this kind of scale), and appropriate network architecture to support the service(s) involved.

As for backup, that's a larger separate discussion, and there are tape drives that can backup that much data quite rapidly. But 80 GB is not insignificant.