Hellcore Mailer allows locking users with passwords, so accessing your data without supplying proper password will not be possible, this way you may protect yourself against sending/receiving your mail, accessing QuickCache or simply running program by others. Current implementation of user lock does encrypt main program configuration (NOT folders) with password, so losing your password may cost you most of your data. Note that folders aren't encrypted (you may do this separately, read more), so they still may be viewed with text editors. Also note that this facility should not be used on extremely sensitive data, for this purpose use tools like TrueCrypt.
To lock user with password, use "Change user password" from "File" menu. If user already was locked, you'll need to re-entrer old password. To remove lock, enter empty password and click "OK" (clicking "Cancel" will not change any password). Remember that password cannot be recovered.