Building a Scalable Mail System?

clusteredMail asks: "I work for a small ISP that up until now has survived with single servers for most critical roles, including the mail server. We are planning to introduce multiple mail servers (primarily for email collection via POP3 and IMAP) and want to put in place the most scalable, resistant to failure system that we can manage. Everything is currently running on one or another flavour of Linux. In my mind, the ultimate scenario would be to have some sort of distributed/clustered file system between the multiple machines, so that any user could log onto any server, and the loss of a single server would not cause downtime for any group of users. Has anyone in the Slashdot community had to put together a system like this using Linux and Open Source Software? If so, how did you fare and what were the major stumbling blocks?"

"So far, the plan is to split up the mail accounts between multiple servers and use some sort of connection proxy to sort out which account logs into which server but this seems like a rough approach. The disadvantage to this setup: if one server fails all the users who have accounts on that machine will be in the dark, email-wise."

The trick is...

[Quaoar]

...to make the mail modular so that people of different sizes can wear the armor without the need for re-forging.

There is always

[jellomizer]

You can Point you MX record to google gmail service and modify the pop addresses to point to google mail too.

open SMTP relays

[JeanBaptiste]

cheap, reliable, scalable, and there doesn't seem to be any shortage of them.

now if I could only figure out how to receive...

Re:Layer 4/7 switching

[fimbulvetr]

This way is the only inexpensive, noncommercial software, non-kludge, non-propretary like way to do it.
I've built several systems like this, one is close to 100,000 accounts with no problems. This system scales out (i.e. adding another cheap server for more power) as opposed to scaling up with huge servers (price, power demands, price, and price).

It's also very easy to troubleshoot.

Do not split your email users between servers and proxy them. Big problems.