- FAST. Mutt lets you process thousands of messages in short order. Mutt is directly responsible for helping me dig out of a 50k deep email hole brought on by years of GMail's approach (archive, never delete) in a few days of on-and-off cleanup.
- Plays well with others. Uses a bog standard Maildir format that anything sane can read (including Thunderbird, so migration should be easy), provides a very powerful hooking system which can do anything from verifying PGP signatures to checking your spelling before sending to displaying attachments.
- Sane defaults. Doesn't require a lot of in-depth customization to provide the basics, and the bells and whistles are hardly out of reach.
- You have a backup of your mail for free (by using OfflineIMAP)
- Not hard to learn (at least for the crowd here). If you can use Vim, you can be using Mutt at full speed in less than a week.
- Search capabilities that blow nearly every other project out of the water (regexes with some very cool niceties)[1]
- Secure by design. Mails are rendered as plaintext, HTML is decoded by piping messages through a program like w3m. Tracking bugs can't do their job, spammers can't see that you've looked at their stuff, and you're immune to whatever image decoding bugs crop up. (Though you can still view images, it's an explicit process). Also, builtin GPG support.
I followed Steve Losh's guide[2] on setting it up.