For the last several years I have been using Google Mail for the majority of my email needs - chiefly because I can access it from wherever I happen to be. No matter how hard they try though, GMail is never going to beat a traditional thick client application (”thick client” means something like Microsoft Outlook - whereas a “thin client” might be Yahoo Mail).
Many moons ago I used Thunderbird 1.x as my email application of choice. There was one bugbear though - downloading email into the computer sucked it from the server - meaning GMail then lost the messages for future reference while on the road. That has all changed recently due to IMAP though - GMail now supports IMAP, meaning you can treat the GMail servers as you would an Exchange or Notes server - they keep hold of the messages and your client becomes a “view” of the server.
And so (now I know I can use an email application without it wiping out the archive of messages in the mail server) I find myself taking another look at Thunderbird in it’s latest incarnation - Thunderbird 2.
So what can Thunderbird now do that it didn’t used to be able to?
Thunderbird now has some wonderful email sorting and filtering controls. You can group by a sorted column too - so for instance, you can see emails broken into a tree of “today”, “yesterday”, and so on. You can also group conversations as threads - making following topics much easier.
You can now tag messages - which colour codes them in the message lists and is also available for filtering. If you are in the habit of keeping old emails, this could be very useful in the longer term.
You can now save searches - which appear in the folder list in much the same way that “smart folders” do in Finder on the Mac - this provides a bit of added flexibility on top of the wonderful message rules that already exist.
Thunderbird now has phishing detection built in - so those emails from Paypal requesting you enter your password will be flagged as unsafe before you even read them.
Of course all of the above features come on top of perhaps the most stable, secure email application available. It has the best junk filtering of any email software I have ever seen - and it learns from it’s past mistakes too. It’s also worth pointing out that Thunderbird is a very good RSS aggregator too - letting you to keep on top of your favourite blogs and news feeds from the same place you write your emails - nice.
If you have never used Thunderbird, it doesn’t take much effort to switch from Outlook. One thing you might want to do is look at the account preferences thought to change the insertion point of message replies - Thunderbird defaults to start you typing at the foot of a quoted message, rather than above it (as Outlook does). You can make it behave just like Outlook if you want.
Head on over to www.mozilla.org/projects/thunderbird and take a look at it - give it a try. It’s free, and it’s perhaps the best all round communication software on the internet. It was built by the people for the people…

Since you mentioned security, I would like to point out some advantages you missed. Thunderbird supports the Enigmail plugin which makes GPG encryption of email pain free. It also supports Transport Layer Security (TLS) which makes it difficult for third parties to read your mail (ever the to and from headers) while it is being transmitted. Used with a secure email service such as the one offered at http://www.novo-ordo.com, you can keep your email about as secure as current technology allows.
And with Thunderbird, you should not have malware going through your address book and either infecting everyone you know or emailing them random files from your computer.
and tb3 will probably be around soon enough.
Although pointless to mention, gmail has always had a option to keep pop3 messages on the server. This of course does nothing with your sent items, they would be stuck on your computer.
Like i said though, pointless for me to say now they have IMAP.
Have you had a look at Chandler (http://chandlerproject.org/vision)?
It is all about regarding email as part of an information management process rather than part of a serial filing process. Started by Mitch Kapor some time back, it has taken a long time to get itself going but I believe in the premise, and it is still the only game in that part of town (I think).