Plus you can add extensions to Thunderbird...just like Firefox...that will add features to make the client much more robust. Like the AutoCopy extension. Great stuff.
PLUS....and here's the big thing; Have you ever tried going through the process of backing up your messages, address book and all other user data with OE? Hah! If it weren't a nightmare, it'd be a joke.
With Thunderbird...just like Firefox...you simply backup your profile directory. Done.
Seriously....dump Outlook Express. It is Swiss Cheese security-wise and is less than a fully functional email client.
One last bit: Thunderbird also "learns" on the fly. It will see email that you delete as junk and will keep track of domains that send the stuff and automatically move them to the trash bin in the future.