Courier's unique all-in-one data file keeps all your email -- your messages, account information and preference settings -- in one file. It's easy to back up or restore your email, or to move it from one computer to another just by copying one file. It's very handy for snowbirds who have a computer at each end of their flight path.
Courier can import messages from Outlook Express, and also from Eudora and other programs that use UNIX style mailboxes. If you have a bunch of saved email, it easy to move it to courier.
Courier is simply more secure than Outlook Express. The safest configuration is to view messages in Plain Text mode. You can set Courier to view all mail in Plain Text mode, and easily switch to HTML view for a particular message by clicking the HTML tool bar button or using the Ctrl-W hotkey combination.
Courier has an encrypted address book making it more immune to worms. You can disable Java script and other script execution. You can disable Active X extensions. You can disable "Web-bugs" -- graphics which let the spammer know you've read his message.
Courier maintains a list of file types that are commonly used to spread worms and other infected files. If you try to open one of these types of attachments, Courier will prompt you as a security measure to prevent the inadvertent opening of a dangerous file.
The WhiteList filter can check incoming mail's sender against your address book. If the from address in a message is not recognized, the message can routed to a possible spam folder. Courier also integrates with the free POPFile mail filtration system for even more anti-SPAM options.
If you miss being able to use the preview pane in Outlook Express, the security features in Courier mean you can have your preview pane back again.
Attachments are not the only Email risk. Email messages in HTML format can be just as dangerous. It's easy to set up Courier to eliminate that risk, yet still read HTML messages easily after you've determined that they are OK. The configuration process is: