Thunderbird is the way to go.

Whereas mail.app can't handle virtual identities, Thunderbird does so with ease by the installation of the Virtual Identities extension. You can get multiple accounts in mail.app. You can even set it to allow you to send email from multiple addresses, if you PRE-CONFIGURE it and follow certain restrictions. Thunderbird, however, lets you create on-the-fly addresses, learns when to suggest what virtual identities should be used for what recipients and the extension is completely free.

Thunderbird's junkmail filter is rather decent, too. Mail.app has spamsieve, if you want to pay $30. Thunderbird's builtin bayesian filtering as well as spamato are free, however.

Thunderbird has a fantastic extension called Nostalgia/nostalgic which lets you quickly filter messages into certain folders with the touch of a couple buttons. You can get an addon to mail.app which does this, but it involves an annoying pop-up interface to accomplish it. Oh, and like most of the apple software in the world, it costs $9.

Thunderbird also has numerous extensions that let you add reminders and tags and notes and classifications to messages. Mail.app has this as well, but it's another $30.

There are numerous other extensions that make life with Thunderbird far superior. Not having thunderbird interact with the operating systems' address book is pathetic, but it's easily overlooked when the alternative is to use mail.app, spend at least $70 for extensions (not to mention future updates to them) and still not have the same functionality that thunderbird has.

Also, the latest OSX build of thunderbird has a very nice OSX-ish feel to it. Not quite on-par with the way the mail.app interface looks, but if you're more concerned with functionality, there is nothing in the thunderbird interface that would detract from the experience.

I started using Thunderbird, and just love it. I added Lightning for my calendaring, with the provider addon so I can sync my calendar to Google Calendar so I can sync it back to iCal and ultimately my iPhone...Works fantastic. Sugarbird is great for integrating with SugarCRM, and with a couple hacks to the iCal plugin I can synchronize Lightning with SugarCRM. A couple more hacks you can find online and TB 2.0.0.6, I am connected with the mac address book.

I use the free Plaxo service to sync the built-in Thunderbird address book with the Apple Address Book. Works beautifully!

After years of development this app still does not connect to the built-in address book in OS X. It is therefore utterly useless to me.

I was a devout Thunderbird user on Windows, but the Mac build plain sucks. I have a lot of mail- my POP account has got to be over ten years old so I get a crazy amount of spam. Thunderbird can't deal with it very well. It's slow and locks up too much. However I don't feel like transitioning to Mail.app or paying for a client.

Is there no such thing as a good email client for Mac OS X?

Mail failed to quit and ate 100% processor for the bajillionth time, and I finally switched.
And WOW, is this better than Mail.app... everything about it sits happily and smoothly in my dock 24/7, where Mail.app had quite a few quirks and outright bugs.

I tried to switch to Firefox, but until now it's been noticeably slower than Safari. Now... I may finally switch that as well.

a nice replacement for entourage, a better handler of muliple accounts too

Started using this with my windows box and had a nearly flawless crossover to my Mac machine. This is a really great mail program. Highly recommended, especially for multiple accounts.

Has some bugs, but is way more functional with IMAP mailboxes than Mail.app, especially over slower links (eg via VPN).

The best usenet reader for os x.