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I just discovered a great Linux mail client called Sylpheed. It's powered by GTK, so it integrates nicely with GNOME apps, and it works wonderfully. I was looking for something that wasn't Evolution (it crashed too much, and is just too big), wasn't Thunderbird (it royally mangles IMAP in an attempt to implement Trash, and it's too user-friendly for me ;-)), wasn't mutt (it is just a little cryptic for me, and doesn't have a nice abstract folder tree), and wasn't telnet (you've never watched me check POP3 mail with Telnet? oh, it's fun.)
Sylpheed is nice. It's fast--it handles a folder with about 2500 messages with ease, and is giving no signs of slowing down. It filters messages as they're downloaded (it doesn't lag behind) and filters that same folder of 2500 messages in about 5-10 seconds. (However, I wish it had a progress bar when filtering an entire folder.) This is important, since I tend to keep large archive folders and don't want to resort to the practice of artificially grouping messages by date. It has good PGP support as well--important for any geek who's trying to "stick it to the man", and for people who actually have secrets to keep, too ;-). Many GNOME programs seem like "the quintessential GNOME _____ program"--the program that looks perfect, has just the features you need, and just works. Sylpheed so far seems like the quintessential GNOME mail program.
I've also tried sylpheed-claws, which is a "next-generation" version of Sylpheed. It's nice, but a little rough around the edges, and not as elegant.
Related: Mutt, a text-based email reader. I have since decided that it is not quite as "cryptic" as I thought it was when I was using Sylpheed, and now use it as my regular email reader.
Mutt is a "small but very powerful text-based mail client for Unix operating systems". It is my current mail client, and my favorite so far, for several reasons.
First, mutt is simple. It's text-based, which makes it a breeze to run over ssh; it includes only email features--no calendar, no to-do list, no other useless PIM features--so it's easy to use and streamlined.
Second, mutt is insanely flexible. It supports a bunch of mailbox formats (mbox, mmdf, mh, and maildir), as well as a bunch of remote mail protocols (POP3, POP3+SSL, IMAP, IMAP+SSL), and allows flexibility in moving messages between these folders (it doesn't assign explicit meaning to any folders unless you ask it to). One major feature contributing to mutt's flexibility is patterns, which allow you to select messages based on many different criteria. Once you have a pattern, you can use it to colorize, score, or automatically process messages.
Like many Unix mail clients, mutt doesn't give you everything in one package. If you want to read HTML mail, you need a text-mode web browser. If it's listed in mailcap, mutt will automatically use it. If you want to send mail (!) you need a MTA. One good choice is msmtp, which is a mail sending program that is as simple and flexible as mutt. Mutt also can't receive mail by itself, unless you point it to a POP3 or IMAP mailbox. I recommend Fetchmail to download your mail and Procmail to sort, filter, or otherwise play with your mail. Mutt shines as an email reader, and integrates wonderfully with the additional tools necessary for email use.
Related: Sylpheed, a GTK+ mail client. Sylpheed is a full-blown GUI mail client, but is still very fast, and has a similarly great set of features like mutt.
