I've switched from Outlook Express at work to using Mozilla Thunderbird. The reason behind this was a simple one: I was sick of Outlook Express marking emails as read and then arbitrarily marking them as unread at random intervals. Even emails that had been on the system for days would suddenly pop up as unread every now and again.

So I've swapped and I'll never go back! Thunderbird is a lovely mail client, it has a nice clean user interface and sports a whole host of nice features. Best of all, once an email is read it stays read.