We use MDaemon (currently ver 10.0.1), we pay for the maintenance which means we get the upgrades free. We use it for all staff and students, which puts us well over the "ultimate" version or whatever it is they call it.
We have a few users on the Outlook Plugin (which costs extra), so that they can use Outlook, but only from within the school. This uses IMAP, so the messages are left on the server, but makes everything appear as Exchange to Outlook.
Other users just use the WorldClient web client, and a lot of the staff use the Comagent software that auto-logs them on, and allows them to send IM messages to eachother.
There's a 20% discount to education.
The annual maintenance is 20% of the cost you pay (so 20% of 80% of the quoted price).
The only problem I find it the
SLT have massive amounts of email messages that they don't read or delete, and I'm averse to giving them no quota.