Thunderbird's been pissing me off lately. Thunderbird3 feels buggy to me, and is constantly screwing up my read/unread messages. Not to mention it's slow and has other odd/buggy behaviours.
It finally got to the point yesterday where I decided to find a new client. After awhile, I installed sylpheed. So far, I'm happy with it. The only issues I've had so far are:
- It polls IMAP instead of letting IMAP deliver
- It doesn't display HTML -- it renders HTML messages as plaintext.
Other than that, I'm really happy with it so far. Compared to Thunderbird, you get a lot more control, customizability, and speed.