I hate mail clients. Really there's no good one I've found yet.
Yesterday I got sick of Mail.app spinning disk for no good reason whatsoever. Yes OK, so I have an IMAP system with several gigs of mail in it, but pine copes just fine with it. When Mail.app is chugging I can run fs_usage and see that it's just a blur of "Mail" entries.
So first I tried Opera Mail. That's OK-ish, but it's so integrated with the browser that it doesn't obey your default browser setting for links, and had a number of other weaknesses that I couldn't live with (like it didn't like hierarchical folders in IMAP).
Then I thought I'd give Thunderbird another go. I actually use this as my PC mail client when I'm forced to use Windows, so I figured I'd give it a go.
My mail client has to be able to do a few things that I've become used to:
1) Sort threads by most-recent-arrival. This is absolutely critical - why would I want a new mail arriving to be attached to a thread WAY up in my scrolly view? Now ThreadBubble promises some of this, but it requires you sort by Date and not arrival time - which is utterly broken since so many systems produce incorrect dates on mails, that when I enabled it there were a bunch of completely out of order threads at the bottom of my subject pane. Plus it didn't seem to entirely work. For what it's worth, the feature I want is a more than 3 year old bug in Thunderbird, and not even due for 3.0a1.
Oh, and if it could do Mail.app's method of grouping by thread but just showing in arrival order that would be bliss (alpine now offers this).
2) Integration with iCal and calendar invites from my windows-using colleagues.
3) Cope with going offline without nagging. Due to my VPN/Firewall situation at work, things regularly disconnect, so it needs to cope with that without nagging. This is currently a showstopper with me for thunderbird.
4) Good fast search. For some bizarre reason Mail.app's Tiger version removed the "Subject or Sender" search. Now you have to choose which you want. That's simply not efficient. Thunderbird's is nice. But their full text search isn't fast. Integration with Spotlight would be good, but the support for doing that in Thunderbird is buggy, apparently.
5) Nice drag and drop UI - this makes it easier for me to drag missed spam into the right folders (they are categorised into "regular spam", "foreign" and "nigerian", so having to type these folder names every time (a-la pine) is a pain.
6) Support for a mixed proxy environment. I have two accounts, one requires a SOCKS5 proxy (that can go up and down all the time) the other doesn't require a proxy.