A small enumeration of mutt's advantages in my experience:
- It's small, fast, and light. I have a mail client that runs in a terminal window (or console session, or remote SSH session). Navigation, search, and controls are all keybindings. It's fast to fire up, compose, edit, navigate, filter, and send.
- Threading. Mutt's thread-handling is second to none (though strongly influenced by newsreaders such as tin and slrn). Threads may be expanded, collapsed, marked read, navigated, joined, and split. Gmail's "conversation view" is a pale imitation.
- The "limit search" feature really rocks. If you want to find a mail from, say, the 14th of the month forward, from bigboss, subject "widget": "~l ~d 15- ~f bigboss ~s widget". You can filter by date ranges, add either AND or OR keyword matches, search by text, etc.
- Once you've identified a set of messages to look at, you can scan through them very quickly via keyboard navigation. Very useful if you're dealing with a large number of automatic / system-generated mail, though others can be handled as well. Literally: scan several hundred messages in a matter of seconds, looking for major differences.
- Tagged message management. Messages may be tagged ('' key), either individually, or using filter/match rules. Say you've looked at all your Nagios alerts and want to move them to an archive or delete them: "T.;s". Much faster than the equivalent in Gmail or any other GUI client I've used. This with limit search is the killer feature for me.
- In-line handling of MIME attachments. There are console-mode readers for many document formats, including HTML, Word docs, spreadsheets, and presentations. Even if they don't give you a fully-formatted view of the document, you can get the gist of meaning without firing up a full-fledged GUI app.
- Spawning MIME attachments. If inline isn't sufficient, there's the attachment index (for mails with attachments) and you can launch these using the handler of your choice (specified by your mailcap file). Attachments may also be saved from the index individually or collectively.
- Adding attachments. Similar to the MIME viewer -- this is actually faster and more flexible than most GUI tools (in particular, Gmail). You can also add messages as attachments, not just files.
- Speaking of GMail: if you do want to view a message in its GUI wholeness, just pop open a browser and view it there through GMail. IMAP means you've got a synchronized set of folders locally and remotely. You can also share IMAP access with a local GUI client (KMail, Thunderbird, Evolution, etc.). This is particularly helpful for calendaring (I'm not aware of a MIME handler that will add ical event to a calendar).
- Comprehensive PGP support. It's highly unappreciated and underutilized, but both encryption and signatures are supported. At the very least you can confirm signatures readily on security bulletins from most major Linux distros.
- Incorporated address books for completion. Typically via an included 'aliases' file. Start typing a name, hit 'tab', and a screen of completions appear. Do this right an you can grab a list of related names in one pass. You can also alias lists of addresses you use frequently (say, "family" or "engineering team" or "zombie apocalypse task force").