James is mail server written in pure Java and developed under the auspices of the Apache Software Foundation.
Is it stable enough for handling large (unpredictably large) amounts of mail? Can it be used by a hosting provider without fear of losing data? Are there any limitations?
I wouldn't touch it with a barge pole. The one customer I've had using it had nothing but problems with it, frequently dying and causing problems with other things running on the machine. There are plenty of other, less sucky, MTAs out there.