I've been annoyed for some time with Outlook Express. It stores messages in a proprietary binary file format that nothing else can read (there are some tools on Freshmeat, but none of them claim to work with OE 5.0 or later, so I'll be transferring my mail archives via an IMAP server), its mail filtering is dumbed-down and overly simplistic, and it has some very annoying habits when it comes to displaying non-HTML messages (for example, it routinely ignores leading whitespace on a line, so code samples, particularly Python, are unreadable; also, it likes to insert "file:" in front of anything that it thinks is a file URL, including C++ comments beginning with //). I also just want to do as much of my work as possible on Linux, even though I make my living writing software for the Windows platform, because I'm tired of Windows crashing and I want my mail on a machine that's turned on 24-7.
So the next question is, which Linux MUA should I use? Well, I like GUIs, and I need access to multiple independent email accounts. These requirements rule out a few clients, such as Pine and Mutt, which aren't graphical, and Balsa, which seems to support only one account per user. I have GNOME, but not KDE, which probably rules out a few more options. My new mail client doesn't have to be GNOME-based, although use of GTK would be nice, as it's my favorite X toolkit.
Since I browse using Mozilla 0.8.whatever (build 2001032212), the obvious first thought was to use that for email too. Wrong! It looked okay at first, but having set it up with all my mail accounts and downloaded my 25 MB of archives from the IMAP server, I now find it no longer works. Attempting to start the Mozilla mail client results in Mozilla apparently trying to consume all available memory and CPU cycles, without any mail client window ever appearing. (I killed the process once it had consumed a combined 150 MB of RSS and swap space -- this on a machine with 128 MB of physical RAM). Now, this is obviously just a bug, but right now, Mozilla is utterly useless for me as an email client.
Another obvious thought was Emacs, since I use it for other purposes already. But although it has an X interface, it isn't really graphical, and since it's single-threaded (at least the FSF version is -- I'm not sure about XEmacs), this effectively means your text editor will pause every time your Emacs mailer decides to poll for new mail. Usually, this won't take long, but it can if there's a problem connecting, or if the server is slow. And I'd rather not have to run two copies of Emacs.
So now I'm looking for other candidates. A quick survey of Debian's package list and www.gnome.org reveals a few interesting possibilities:
Evolution is Ximian's forthcoming Outlook killer, integrating email, contact management, and personal scheduling. The feature list looks superb, but it's still in a relatively early stage of development, so I'm not sure I care to trust it yet. Does anyone use this for their primary MUA? Is it ready for such use? (Their own web page says it's not, but that could be considered just a legal disclaimer.)
Mahogany is GTK-based and has a built-in Python interpreter, which is a definite plus. Its web site doesn't say exactly what you can do with the Python scripting, but I would guess that mail filters can be written in Python, which would be a vast improvement over Outlook Express's simple filtering. As for multiple accounts, well, I'm not quite sure. The web site says Mahogany supports "multiple incoming mail folders", which might mean multiple accounts.
Cronos II is another GNOME mailer. It supports multiple accounts, but does not have IMAP support yet. (I can live without that, actually.)
MMC is also a GNOME mailer. It supports POP3 and IMAP. Multiple account support is planned, but not yet implemented.
Pygmy is another GNOME MUA. One of the first things I found out about it is that it's written in Python, which leads me to suspect that it will be slow as hell. Additionally, it has no POP3 or IMAP support; you have to use fetchmail or some similar program to get your messages from the server and put them in your mailbox.
I would be very interested to hear about anyone else's experience with any of these mailers, or about any other mailers that anyone uses regularly.