Help: Apple Mail losing messages

After using Mail for a couple of years, I have been having recent problems with it losing some received messages, but the most pronounced problems are that it has lost many sent emails, some drafts, and it no longer keeps forwarded messages that I have appended. I've tried the rebuild mailbox command to no avail.

I've also tried recovering them with Time Machine by opening the mailbox the messages should be in and then entering TM. No messages are retrieved. Any ideas on this would be appreciated. This an extremely serious matter for me because I depend on a record of messages I have sent and received.

Is this a known problem with Mail? Also, I'd appreciate recommendations for another solid email program besides Entourage.

Mac mini 2.26 GHz Intel Core 2 Duo; OS 10.6.6; 4GB ram. The machine seems stable otherwise and I run Intego's Virus Barrier, which reports no problems.

Any help is really, really appreciated.

Thanks.

 

It's POP.