Thoughts on the best, advanced email client?

Which is the best Mac email client for advanced usage?

I presently have Mailsmith, but it just crashed and I lost all of my files. It appears this has to do more w/ my hard drive than Mailsmith itself. But I’m now reconsidering what kind of email client would be best, esp. since I’m about to upgrade to a new powerbook and Leopard. The most important emails features for me include:

-Searching (critically important)
-Filtering (I had about 12 mailboxes for one account, so smart folders are essential)
-Stability (crashing and losing email is disastrous)
-Security (wish I could easily improve, but such functions seems elusive)
-Capacity (for storing lots of email)

Powermail looks like it has amazing search tools, but I’ve read that it’s lacking good security.

Thunderbird has a new feature that enables one to look at the viewing history, just like a web browser. That looks appealing, but I don’t know much more about it beyond that feature (and I've heard negative things about it).

I've heard good things about GyazMail, but would be more tempted if they offered better security and Spotlight support.

I’ve heard positive things about the Mailsmith’s beta, but have advised not to use it for important email since it’s still being beta tested. It’s unfortunate since they’ve been testing it for almost two years now.

I have no idea which email client offers the best security. I have PGP but haven’t even really used it since it’s hard to find others who have it and use it. I wish I could find a better solution for security, esp. when I travel.

Any thoughts? Suggestions?

Still searching for that perfect Mac email client, indeed...

Thanks!