I have spent the last two years feeling slightly disappointed with Mail applications for the Mac. While there are not a lack of choices, I do feel that there is not one strong candidate who can get the task done the way I would like to see it done.
Mail.app
For the most part, Mail.app fits the bill as a default mail application, and it is the most well-rounded of the bunch that I have used. It is also my default email client and the one that I feel most comfortable with. However I always find something is lacking in the feature set it offers. Specifically, I am disappointed with the search functionality it offers, the obvious lack of multiple search fields and or a Boolean option are difficult to overcome when I have nearly 15,000 emails in my work account alone. (Yes, I know of Inbox Zero, haven't gotten there yet, but only half of those are still in the Inbox. However, when I do need to reference an email, Search would be the way to go.) Also, I would love to have tabbed-browsing for emails, since the feature was released, it has brought so much order to the chaos that use to be window-browsing. There are times and emails that I cannot reply to immediately, but need to pull up four other conversations, documents, and sites as reference and then collate into a reply. That is the beauty of the tab, it's not cluttered like having windows all over the place, yet it is still there to be quickly accessed and drafted. I hope that Apple will integrate tabbed-browsing soon. However, Mail does do one thing that has saved me quite a few times, it re-opens all the windows (messages) after quit or a crash. That my friends, has saved me from epic disaster, and something that the king of email Outlook does not even have (To my knowledge).
Plugins:
Plugins are a great feature (hack) for Mail.app. I use a lot of them. My issue with most plugins is this: I use Mail.app for email, I use an iPhone for email, I use the web interface of Google for email (*sometimes*). I want the information from one interface to seamlessly and effortlessly be available in all. *This isn't necessarily Apple's fault, it could be the plugin developers, however this is a flaw with the design.
Mail.app is the best all-around contender in the lineup, however there are drawbacks, especially for heavy e-mail users. It is also very easy to become relient on Quick Look and the calendar integration for quickly adding events, the majority of the other clients lack anything to counter these.
Postbox
I tried this during its beta and up to version 1.1. I liked it for the most part. Tabbed email view is an outstanding feature, it (like tabbed web browsing) just makes sense. Too many tabs can be detrimental to productivity or to actually dealing with something. Search operators are nice, but they should be universal to all interfaces. I won't go into the nitty-gritty of it, but bottom line for me: It feels like Thunderbird.
Thunderbird
As of version 3, Thunderbird is now native. It still doesn't look or feel native.
Mailplane
I go back and forth between Mailplane and Mail. Mailplane is an application that interfaces with Gmail. It has some nice features like being able to setup multiple accounts, integrate with other OS X Apps, Services menu, and you are in essence in Gmail, so any interaction from a web browser would properly reflect accordingly, but when I do switch back to Mail.app, my message replies/forwards are not linked. It is very usable and clean. It doesn't have tabbed emails, but you can open up emails in new windows. One issue I have is the handling of multiple accounts, however, I am not sure there is a fix. When you switch accounts, any open windows (emails) will be closed, not a big deal as long as you know ahead of time, but sometimes I need to pull info from one account while drafting from another. Obviously, this can be worked out, but from a process optimization standpoint it can slow things down.
The one feature that keeps me going back is Google's search. It is glorious.
Entourage
This is the email client of my workplace, and Microsoft's horse in the race. After many tries with it, I just found it couldn't do everything I wanted to do. Due to the overwhelming shadow of Outlook that Entourage is often lost in, maybe the expectations were just too high. It now appears that MS will be bringing Outlook to OS X in the next release of Office.
This could be a very good decision, or it might backfire entirely. In my opinion, it is most dependent on the Integration aspect with OS X, specifically iCal and Address Book, as well as the design of the UI.
Overall
There are mail application options. The strongest contenders in my opinion are Apple's client and Mailplane (web). The second is based on gmail, so it would vary for other web-based email clients. However, there is a lot of room for improvement, and that is why I am excited to see that Brent Simmons has opened it up with his post on email:
http://inessential.com/2010/01/16/email_init
I look forward to seeing what Letters comes up with as well as what MS does with Outlook. I feel that we are at the same state of the email applications that we were at with the client-side vs. server-side mail protocols just prior to full-blown IMAP adoption. Things are moving to the "cloud" or on a more practical scale, to handsets (iPhone, iPad, Android...). The key to all of this progress is to have a unified interaction across multiple platforms that reflects in all.
