Fast e-mail client for OS X?

Solution 1:

Nobody's mentioned Apple Mail. I love it, particularly the Lion version.

Solution 2:

I've switched full time to Sparrow largely because of how well it handles multiple email accounts. I'm willing to put up with some of its other quirks because of the fast account switching and unified inbox view. Switching between accounts is the fastest I've experienced of any tightly-coupled-to-gmail mail application I've used on any OS and the minimal UI appeals greatly to me. It's support for non-Gmail or non-IMAP accounts is only so-so right now, but if you fall within those use cases it's definitely a contender for a great mail client on OS X.

Sparrow does download your mail to local disk; it's part of what makes account switching so snappy (versus something like MailPlane where it's a UI wrapper around Gmail's web view). Between my 3 main Gmail accounts I've got just under 18,000 email messages for a total of ~3.5 GB of space on disk consumed by messages and their attachments. That should give you a baseline for comparison to your own account volumes and needs.

It stays in lock step with Gmail so I don't worry about backing up my local Sparrow directory with Time Machine -- Gmail has copies of everything and gives me the ability to undo any horrendous mistakes I might make with my mail accounts.

Solution 3:

With IMAP it seems that MailMate is extremely fast and powerful, as already suggested. I'd try it out, to see whether it suits your need. I tried it once, but the poor gmail support at the time had made me drop it.

Then there are a plethora of without-GUI apps, like mutt, which is highly praised by the UNIX-folks.

If you need a client with robust Exchange support, you can't go wrong with Outlook:Mac 2011.

But, just for the fun of it, why not trying Mail.app?