Claws Mail

Started using Sylpheed a whole bunch of years ago, after following an odyssey searching for a visually appealing, elegant MUA to handle vast amounts of mailing lists (i.o.w. do sophisticated sorting) and support multiple POP3 accounts. Things moved, Sylpheed-Claws branched out of Sylpheed, became Claws Mail, which now is to be considered "mutt-on-X" in some ways. Maybe in some visual ways it is less appealing than, say, Thunderbird these days, but its feature set is just what you want being an "advanced" user with a load of e-mails to deal with:
- The filtering still shines, especially the ability to "automatically" generate filtering rules from mails (which worked out almost every time I tried, so far).
- Folder post-processing is a dead cool feature to all those who don't want to have old messages just deleted but rather moved to some archive.
- The ability to "watch" communication threads keeps things manageable even while trying to keep track of many conversations on a bunch of list.
- Inclusion of external tools (most notably bogofilter) provides a decent security from being plagued by spam and friends.
- A bunch of useful plugins do good things to you, most notably in my case vCalendar (which works well exchanging appointments with Outlook 2003 and Thunderbird clients in our company) and RSSyl (for reading news feeds, obviously). Claws, looking at the plugins, also knows how to deal with the dreaded winmail.dat attachment you still stumble upon once in a while.
- Compared to Thunderbird, it's very light on resources (actually, this is only likely to change if you throw in RSSyl, but that also doesn't make things better on Thunderbird).
- Asides being "GUI", it's just a tool rather "Unix": You can virtually replace most if not all "functional" components of the application, use fetchmail and procmail and your local MTA to do the heavy lifting of transporting, sorting, delivering mails and just use Claws as a front-end to this mess. You might throw in your favorite external editor if you can't live without vi or whatever. You even can have tools like "fortune" automatically generate signatures for you, which also is rather nice a thing to have.
- Speaking of signatures: The fact that user can choose whether or not a signature template automatically is prepended dash-dash-space is pretty helpful as well, once in a while, allowing for "abusing" signature mode as a small-time templating system. ;)

Overally... tried others once in a while, and always switched back again. An almost perfect application. Minor drawbacks:
- I miss the maildir plugin which used to be around earlier but somehow just seems to have vanished as time passed.
- As most MUAs (including TBird), Claws has some shortcomings dealing with IMAP, most notably spam filtering and mail sorting (which, however, comes as no surprise looking at how IMAP filtering is likely to work).
- Spam-filtering could be faster, although this is likely to be a problem of pushing mail through an external tool. The only really annoying thing about this is that, even up to 3.3.1, the folder list just gets updated (to show where "new" messages have been dumped) if _all_ mails in one account have been processed. This can take a while, back from three weeks of vacations... ;)

Definitely an application to be recommended, however. Wouldn't want to miss it.