aD is now providing a web mail solution in the latest ACS, and it is in the process of being ported over to openACS. Postgres isn't the problem, it's the fact that aD wrote it in Oracle's embedded Java (SQLJ). Dan Wickerstrom, who's doing the port, has come up with a general solution that should allow folks to write Java code and stuff it into AOLserver ala Tcl, etc. Much better than the SQLJ kludge for the greater AOLserver community, if you believe Java to be the Right Tool for You at least.
I don't think our Postgres users are thinking on the AOL/Hotmail scale of service, yet at least. There are potential scalability issues other than the db - after all, the JVM for x86 Linux is interpreted rather than compiled and that's an order-of-magnitude pig to begin with compared to a solution built on (say) C routines embedded in AOLserver, or native-compiled Java.
