Comment on Thunderbird

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.