I have just tried my most complete dalliance with Thunderbird, the free open source mail reader by the same organization that brought you my favorite browser,
I had looked at Thunderbird before, but had decided not to use it because it has no calender. This was especially important in therms of my PDA, which I would sync up with Outlook. There is a calendar extension for Thunderbird that is quite popular, although at the moment on the Thunderbird site it has a low user rating because so many people are angry that it hasn't been upgraded to be compatible with the latest versions of Thunderbird. But it wasn't a complete solution ayway.
I decided to give Thunderbird a try after my PC died and I went to a backup. While I was using a different hard drive and thus didn't have all my saved mail, and since I have taken to carrying a Nintendo DS instead of a PDA, I figured what the hell. When I got my old hard drive (in my backup PC) I started using Outlook again, but at some point when checking memory usage with the task manager it looked as though Thunderbird used less memory, and since I only have 256 MB in my backup PC memory is at a premium. So I actually imported all my mail from Outlook into Thunderbird and used it for several days.
This comparison is between Outlook 2002, because I don't have 2003, versus Thunderbird 1.5, which is actually not quite released. If you go to the website you will see they only have an earlier version. I can't recall where I found 1.5, they don't make it that easy to find but I found it somewhere, probably through google. There's actually a 1.6 Alpha version floating around now that I haven't tried.
First off, I was wrong about the memory. Both programs are constantly taking memory and giving it back. When I first had them running I saw Thunderbird at about 20,000 KB and Outlook at 40,000, plus I had a program called K9 running to deal with spam in Outlook, and that was another 9,000. But as I kept testing I found that both programs would, depending on what they were doing, go up to around 40,000, and while Thunderbird would drop down to 15,000 from time to time, Outlook would, when minimized and not doing anything, drop down below 10,000. The memory issue became even less significant when I decided to see what other people thought of Outlook versus Thunderbird and found a very good rundown that mentioned an anti-spam add-in for Outlook called SpamBayes that can be installed into Outlook. This meant I no longer had to run K9.
Thunderbird has good internal spam filtering, which now also includes support for SpamAssasin, which helps evaluate spam somehow. Outlook 2002 has some simple junk filtering but it's not good for much, but with the discovery of the free add-on SpamBayes I now have integrated spam filtering in Outlook. I just started using SpamBayes, so I can't say how good it is, but it seems promising so far. Unfortunately it lacks support for anything like SpamAssasin and also lacks DNS blackhole list support (these are lists of known spamming domains). (Thunderbird also doesn't support the DNS lists.)
In terms of stability, Thunderbird wins hands down. Outlook has always tended to freeze and crash on me. To make matters worse, any time it crashes, the next time you start it there's a diagnostic process that, with all the mail I have, takes several minutes, during which time I can't use Outlook. This is typical of Microsoft products, which always approach any system crash by doing something annoying that takes a long time. It often does this recover thing even though it was closed normally; sometimes Outlook just gets into a state where it always feels it crashed and starts up with the recovery thing every damn time. Thunderbird is less prone to crashes and freezes and if it does crash it doesn't take extra time to restart.
Outlook by a mile. Outlook supports very elaborate filters. For example, I belong to a Freecyle New York Yahoo group from which people can offer stuff they're throwing out. I have a filter in Outlook which looks for freecyle emails with offer in the subect (you can also post "wanted" requests, which I filter out and which also has one of a number of items I'm specifically looking for (a receiver, a disk of Outlook 2003, etc.). This can't be done in Firefox, which has a much less sophisticated filtering system. The best I could do was create a folder for all freecyle mail (with wanted filtered out) and then create a search folder that searched for items I want. A search folder is a neat thing where you can create a virtual folder that looks for specific things, for example, a search folder that shows all your unread mail. Back when I used to use Entourage, Microsoft's vastly superior mail reader for the Macintosh (for some reason Microsoft's Mac division makes better software than their PC division, at least in terms of Microsoft Office), I could see all my unread mail or flagged mail at a glance, but in Outlook 2002 there's no way to do that. On the other hand, I just learned that Outlook 2003 has their own search folders, which is why I'm hoping someone will give away an unwanted disk of it on freecycle. Thunderbird's search folders are imperfectly implemented so I'd be curious to see how Outlook does it.
Outlook has really good datebook integration. You can actually drag an email into the calendar folder and it will pop up with an appointment with the email in the body, which I thought was incredibly neat when I stumbled across it. The Calendar is really good, you can easily drag appointments to different dates and other good things. For Thunderbird 1.5 all there is now is ReminderFox, a very basic extension that keeps track of appointments. It's an admirable attempt but rather clunky.
I like the way you can create and manage multiple address books in Thunderbird. I could make a friends address book, a business address book, etc. and populate them from my main address book. This was useful because I could then have a friends mail folder and just say, any mail from this address book should go to that folder. You can create multiple address books in Outlook 2002 too, but it's a pain and you can't work with multiple address books at once, so there's no way to divvy my existing addresses into new address books. So while if I had a populate friends address book I could indeed use it to send friends emails to a specific folder, it would be hard to set up (right now I just filter for individual senders). On the other hand, Outlook has categories, which is handy. If I could filter for email from people in specific categories I'd be all set, but I can't.
Thunderbird also doesn't have a built-in way to add addresses to an address book. There is an extension that does it, but it doesn't seem to be good at recognizing when you're putting in a duplicate, although I only tested it a little so perhaps it's better than it seemed. It's also not been upgraded for 1.5, but another extension called MR Tech allows you to force install extensions, and doing that I found the add contacts extension works well except for the duplicate thing. But Outlook does a better job of adding contacts.
People claim Thunderbird is more secure, and I have no idea but I thought I'd just mention that. I have Outlook set to be careful about attachments and not run anything when I open an email, so I haven't had any security problems, and I suspect the people concerned about security are as careful as me and aren't really having problems with Outlook either.
As much as I would like the free, open source Thunderbird to be better, it is trounced by the corporate giant. I don't know how it compares with Microsoft's free Outlook Express though, so perhaps for those who don't want to pay extra for a mail reader it's a good bet.
I suspect someday Thunderbird will surpass Outlook, but it will take a while. Firefox is so good because so many people are coding for it, taking on additions and creating extensions to allow you to do an incredible number of things. Extensions are a main selling point, something Microsoft has never really understood. Even when I tried looking around for an integrated anti-spam filter for Outlook I couldn't find one until yesterday, because Microsoft doesn't offer a central location for that sort of information. This is very stupid of them; if they embraced the amateur coder community they could have much more useful and powerful products, but Microsoft is a control freak and seems to hate encouraging outside code. Anytime Windows Media Player 10 crashes, perhaps just because the PC crashed, when you restart the player it turns off all your plugins. Microsoft's disdain for add-ons is, IMO, a major failing that will become more and more of a hindrance if they don't rethink things.
Right now there aren't nearly as many extensions for Thunderbird, and I think that's simply because not as many people are interested in using it, so there are less people working on improving and modifying it. But as Thunderbird improves, more people will use it, and more stuff will be created for it. Once there's an integrated calendar that syncs with the popular PDAs, which is being worked on, a lot of people will make the jump. Eventually there will be a tipping point in which Thunderbird is so close to Outlook that open source fans will start jumping over in droves and there will be a huge increase in extensions and improvements, and then Thunderbird will start pulling away from Outlook. But from what I see I don't expect for that to happen for at least another year. But someday, I suspect, I will come back to Thunderbird.