To backup your Postbox email profile browse to
Mac:
/Users/[username]/Library/Application Support/Postbox/Profiles
Vista:
Users\[username]\AppData\Roaming\Postbox
XP:
Documents and Settings\[username]\Application Data\Postbox
And copy the folders named xxxxxx.default to your backup location.
If you have created a new profile then your folder may also be called xxxxx.yourprofilename.
If you just want to backup your email and not your preferences or settings then go into your profile folders and backup the Imap, Mail and sometimes Local Mail folders.