I added POPMAIL to do my email filtering. It's quite good and the price is right (free.)
I have been using Outlook express for 8 years now.
Let me say upfront it is the one M$ product I don't dislike (altho I use several others as they came preloaded with various operating systems). I do have a general distrust of M$ products-- they are quirky, often hard to use, sometimes vulnerable to attack (as OE is).
Things I don't like about OE are security related, but I'm fully patched, have my preview pane closed, etc. And, OE is easy to use, is a nice newsreader (which I use extensively) and generally works fine... for me anyway.
The one thing appealing to me about T=bird is its spam filtering. OE's rules-based filters are fine, but do not go far enough. I've tried several add-on spam filters, but none were satisfactory and seemed to upset other settings, so I gave up.
So, it comes down to this: T-bird's additional security and it's presumably superior spam filtering make it an option to consider. It is not offering me anything else that OE isn't doing fine for me already. Given that, I'd like opinions on whether to go and make the switch and why.