Considering the age of email as a technology, and the plethora of clients available, the situation is tragic. Here is a line-up of the clients I've used the last few years, and the reason I stopped using each and every one of them. Some simplifications are made, such as in the use of words like "cannot". Yes, there are often hacks to circumvent the limitations. No, I won't go into why using a closed source, 4 year old, unmaintained, 3rd party plugin is a bad idea.

Outlook

CERN uses Exchange, so Windows desktops come with Outlook 2003 installed per default. I'm an open source enthusiast, but at first I thought it was a good idea to use what my colleagues used (and continue to use).

Pros:

  • Fast, probably because of close integration with the server
  • Saves message settings (such as flags) on the server, so I can set a TODO for when I get home or reinstall
  • User friendly wizard for filter rules
  • Integrated (CERN only) plugin to report spam and control spam settings
  • Working, simple predefined search folders
  • Good LDAP integration in right click menu

Cons:

  • 32 KB (Update: sorry, the link is dead) to store filter rules is not enough!
  • Resets the reading pane settings for some folders willy-nilly
  • Cannot bounce messages
  • Not enough configurable (show headers, color encoding of fields / replies, etc.)
  • Cannot export settings to human readable file (I want to keep it in CVS)
  • Flags cannot be assigned names or meanings
  • Flags are not the same as in Thunderbird or Mutt, and are thus only visible in Outlook
  • Closed source
  • Cannot use regular expressions in searches / filters
  • Difficult to copy email addresses
  • Bad thread display support

Verdict:
Works for the most part, but is not "techie" enough. I want the open source geek edition!

Thunderbird

This seems to be the most popular email client for open source enthusiasts. I've tried just about every version since 0.8 or 0.9, and the following points are from 1.5.0.4.

Pros:

  • Open source
  • Very configurable
  • Lots of useful extensions
  • Mostly plain text profile - Can be stored in CVS
  • Single click to mark as read / unread / junk / not junk
  • Meaningful labels
  • Supports standard flagging mechanism (same as Mutt)
  • Easy copying of email address
  • Good thread display support

Cons:

  • Bad default training on the spam filter
  • Junk mail filtering can "self-destruct" without telling the user - Empties the training file and never starts learning any more
  • No visual representation of flagging
  • Doesn't react to changes in flagging by other applications (Mutt)
  • Sometimes crashes or hangs on startup
  • Mail filters often break - Client crashes when they are applied

Verdict:
This is not version 1.5. This is 0.9 alpha.

Mutt

I learned to use this textual email client when working for Orakeltjenesten. I could see why the other techies loved it - Stable and geekily configurable.

Pros:

  • Extremely configurable! Examples:
    • Set Reply-to based on To / Cc
    • Include other config files
    • Redefine key bindings
    • Create new key bindings
    • Wanna backup to backup@gmail.com and purge all non-flagged mail older than two weeks with CTRL-o? Add
      macro index \Co "<tag-pattern>~r>2w!~F\n<bounce-message>backup@gmail.com"

      , and just hit d when it's done.
  • Simple and clutter-less interface - This is the iPod of email clients
  • Human readable configuration and address book files
  • Stable as lead
  • Color coding available for everything
  • Very good thread support
  • send-hook . my_hdr X-message-flag: Outlook is bad for you, use Mutt
    - A classic
  • Open source

Cons:

  • Doesn't cache emails
  • *nix only - Windows clients are no longer maintained
  • No sophisticated flagging - Only boolean
  • Doesn't do filtering - Nnnuuuuuuuh!

Verdict:
With filtering and some user friendly front-end, this would be the mail client of the century.

Gmail

Pros:

  • Fast search
  • Only browser needed
  • Settings online
  • Stable
  • Fast
  • Good spam filtering
  • Flexible labels

Cons:

  • Simplistic filtering
  • Closed source
  • Page length limit of 100
  • Max 20 filter rules
  • Cannot backup settings or mail
  • No warranty
  • No extensions

Verdict:
Not enough configurable, and too bad filtering.