I recently switched to using Claws Mail
from Mozilla Thunderbird
. This was mostly to try something new and because of the seeming stagnation of the Thunderbird project. So far so good, here are a couple things that I think are cool.
Lots of plugins
. Claws Mail has for all sorts of tasks and features. The ones that I think are key are GTKHTML, Notification (libnotify/notify-osd) and SpamAssassin. SpamAssassin in interesting because it actually uses a full SpamAssassin installation like you would see on a mail server. The plugin connects to the spamd daemon running on your system and you can teach it what is spam and ham. If you are running Ubuntu all/most of the plugins are available in the normal repositories.
Also very cool is its auto-generated mail filters. Basically you can right click any email and create a filter based on the headers. This worked great for all the mailing lists and ticketing systems I use.
Claws also has a network log which worked great for diagnosing issues with an IMAP or SMTP session.
There are a few quirks, the first that I noticed was that when Claws is checking the mail servers for new mail it locks out some of the menu items. This is annoying if an automatic check happens when you are trying to change a config item. Another annoyance is when using the up and down keys to go through messages you either have to click on the message or hit enter to view it. It would be nice if they loaded as soon as you arrowed to it. Lastly, the interface seems a little less polished than Thunderbird but that may be just because I am more accustomed to TBird after years of use.
If you are interested in trying Claws out and use Ubuntu I recommend adding the Claws PPA
to your apt sources to get the latest version and claws-mail-extra-plugins.