I've used most of the main email clients for the Mac over the past four or five years. Every now and again, something will wind up not satisfying me (none of them are PERFECT, after all - at least not perfect for ME - and if they were, then they almost certainly wouldn't be perfect for someone else), and I'll switch to another client. Usually, I'll be pleasantly surprised to remember some of the good things about that "new" client that I'd forgotten. It's a bit like starting to date an old girlfriend again - you're reminded of all the great things about her.
But it isn't until a while in (usually just AFTER you've given her the keys to your apartment again) that you start remembering the not-so-good things, the things that made you want to break up in the first place. I've had this sort of serially monogamous relationship with various email clients. But I have to say that over time, the one with the fewest glaring flaws turns out to be Mailsmith.
A lot of people make a big fuss over Mailsmith's lack of IMAP support and a few other perceived failings. I look at it the other way: their press release says that they tried to properly implement IMAP, and wound up giving it up because they couldn't make it work to THEIR standards. They don't go into detail, but it's quite clear that they feel either IMAP is less secure inherently, or the idea of being more or less permanently connected to various IMAP servers poses its own potential security risk. Either way, the tone is clear: they're not adopting it because IMAP isn't up to THEIR standards.
Will that drive off some potential customers? Clearly, it already has. Does it make the bigwigs or the developers at BareBones Arrogant Bastards? Yeah, it probably does.
But the funny thing about Arrogant Bastards is that history judges them based upon how right they were, not whether they were arrogant. I kinda look at the brain trust behind Mailsmith as the Jello Biafras of email client developers: arrogant, sure - but usually right, even if it pisses you off a little to admit it.
If you can get over the VERY spartan interface (no customizable toolbars, very little color) and the all-performance, no-frills approach, you will find you've got what remains possibly the best Macintosh email client on the market today - and that's WITH the famous lack of updates from BareBones. Why don't they update it? When you get something right the first time you do it, why tinker? How long was the Lamborghini Countach on the market? Ten years? Twelve? It might not be your kind of car, but you have to admit they pretty much got it right the first time.
What sets Mailsmith apart are three things: the scriptability, searchability and the filtering. Both are literally unmatched, despite some of the reviews which say that Apple's Mail is catching up. It isn't. Filters can be attached to any number of mailboxes in any configuration, restructured so that they run in different orders within each mailbox, you name it. I have no idea how many hours they must have spent working to make Mailsmith as scriptable as it is, but I shudder to think. If you're a tweaker, or someone with large and/or complex email needs, you can do more - FAR more - with Mailsmith than with any other email client out there for the Mac.
Oh, and one other thing: Mailsmith was actually THE FIRST program which came bundled with SpamSieve. Why? Because Michael Tsai at c-command (makers of SpamSieve) probably believed that pairing his best-in-class Bayesian spam filter with the best-in-class Mac email client made perfect sense (just guessing, here - I don't know Mr. Tsai). But the two of them are a powerful combination, indeed. If you're someone who's looking for a lot of neat-colored icons and pretty HTML-rendering capacity, then this isn't the email client for you. But if you're interested in the Mac's best program for the handling of email, then look no further than Mailsmith.