I'm thinking about replacing the mail client. Open Source mail clients are not bad. One of the things they still do not have is keeping the mail secure. But that, in fact, is not the task of a mail client after all. There is an excellent program, called TrueCrypt, that helps with this. It is a program similar to PGP Disk, but Open Source. It's free and secure. Create a secure storage and put all your unencrypted mail from an Open Source mail client on it.
Another feature I miss is a good calendar and Address Book synchronization with Palm. Something might be worked out with Palm Desktop and Thunderbird's Address Book. Palm Sync actually works.