You can keep your Mail.app mailboxes on an encrypted image created by Disk Copy. Mail keeps your mailboxes in the ~/Library/Mail folder, and you can copy this folder over to an encrypted image. Unfortunately, you can't just delete the original and replace it with an alias (by dragging the folder back while holding down the Command and Option keys). That doesn't work -- Mail will not give a warning but will not find your mailboxes, either.
Instead, you must create a symbolic link. Suppose you have created an image which mounts on the desktop as a disk named SECRET and you have copied over the Mail folder and deleted the original. Start up the Terminal application and type
% cd ~/Library
% ln -s "/Volumes/SECRET/Mail" Mail
Now if you look back in ~/Library folder with the finder, you will find what looks like an alias. Mail will now work correctly. By the way, if you are using Eudora for mail (in Classic mode), you can just create an alias to the Eudora Folder. A symbolic link is not required.
You can, of course, use a more generic version of this hint to keep your Mail folder on any volume, not just an encrypted disk image.]