If your looking for a new client you could always give Mozilla's Thunderbird a try. You may also wish to add Mozilla's Lightning extension to the application for calendaring and organisational tasks.

It's comparable to Firefox when it comes to customisability and theming in particular - although the add-on database isn't quite as large. It's also fairly secure when compared to Outlook Express and similar Windows technologies and receives frequent security and stability updates.

You can find a full list of features here.