I use Outlook Express at work. Officially I should use Outlook, but I really can't deal with it. For such a major application, it has a surprising number of flaws.

OE has its limitations though, the lack of filters on IMAP mail is a big one and only being able to search downloaded mail another.

Every know and then I realise how silly I am to be using OE and look around for something else, however I've yet to find a replacement that has an attractive interface (why do all mail apps have ugly toolbar icons?) that also follows the Windows GUI guidelines and is free.

Mozilla has all the features, but I've never been keen on skinnable applications. It is also far too bloated for something that I use all the time.

I've even considered switching to webmail. I'll stop short of using Mutt though.