TreoCentral's
Communications forum may prove of value. Although Treo-oriented, I would think most of the apps would be able to run on practically any 'net-connected PalmOS device.
Products should be available at
PalmGear or
Handango.
Now, there are some good commercial packages that focus on the Enterprise market space. Unfortunately, they're not really proced for the individual. Some are like the BlackBerry system & require a separate server to support the messaging.
The new Treo 650 supports MS ActiveSync for syncing with Exchange on the back-end. The app is actually VersaMail and is apparently available for other PalmOS devices. You might check it out as it does POP3 & IMAP. Unsure about SSL but I think it does.
BizCon (Business Connection) that Sprint provides does POP3 & supposedly SMTP although I haven't gottent the SMTP to work with Gmail yet. The client is really
Seven's product.