I have used so many clients over the years, it's hard to keep track of them. For the record, I only use free ware clients (except one, but that's for later on)
My main requirements are the abillity to have seperated accounts, view HTML mail, and lightweight. Sadly the latter one seems to be the trickiest one.
My first few years on the net, I spent in Linux. Thus I also used Linux clients. I started out with
Balsa, which is a very decent client. It's fast, highly configurable. But there's no support for HTML mail. I found that a real shame, because this is a decent email client. But in this day and age, HTML is used far too often.
After Balsa, I wanted that HTML support and installed
Evolution, which is more a groupware suite than just an email client, it has tasks, a calendar, Contacs. The reason I mention contacts seperate from the addressbook, is because Evolution allows you to enter birthdays, and a whole bunch of info there. Also there's a connection to Exchange servers from within Evolution, there can be a data server set up. Needless to say, Evolution is a pretty big client, and I should probably not use it solely for emails. This is actually more for business people. Of course this client too is very configurable, you can choose if you want external images to load up, HTML or not etc etc. ALso important is a built in junk filter

Next I used for some time,
Netscape Mail. I also used Netscape as Browser, but then quickly changed to Mozilla and
Mozilla mail. Both are practically the same, they just look a little different. Both have HTML support, they're ok, but I found them to be too small. Meaning, too little options. Of course this was over 4 yrs ago. It probably changed a lot by now.
Let's see what else have I used as mail client.
Kmail, from KDE. Which is comparable to Evolution only a bit smaller. I didn't care for it too much. It's now Kontakt, with mostly the same features as Evolution. Although I didn't find anything about the ability to work with Exchange or Groupwise.
Now let's talk about the Windows clients that passed my computer
First I used the awfull
Outlook Express (I was a n00b then), after a lot of trouble, I did switch to
M$ Outlook (not express). Which I also hated. LOL. So my search went on... I tried
Incredimail and actually paid for the blasted thing.
Incredimail was pretty nice. But after a while it seriously gets annoying. Not only that, but for recipients who have dial up etc, it takes AGES to receive the email, since it has a lot of graphics attached. I found the free version of Incredimail spyware-y. The paid version however is cool. What I did love about Incredimail are the notifiers. You can just X out and still have it running as background process. That's a feature that I also look for in email clients, however it is not that important. I mainly stopped using it because Incredimail is not the lightest nicest client for system resources, and it eats up BW (for my internet account I have 10GB BW a month to download and upload).
Now my prefered client for windows is how else could it be :
Thunderbird. For all the reasons I've discussed above. Sadly it is not easy onmy old system either, so I am waiting for my new computer and I'll just use TB again

.
I also tried
Eudora and
Pegasus. I prefered Eudora of the two. But I don't have much experience in either. So a small review is too big LOL. What I did love about Eudora is the Identities, how that works, signatures etc. I found the way to add emails to your addressbook slightly clumsy and pretty damm hard to figure out if you're a n00b. (I test this by letting mom have a go at it LOL)
Now, FoxMail looked nice, I may try it in Win some day
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