There is no reason it could not use a database, but not that I know of. The email messages end up in a directory in plain text. So for instance if you are running MS Server 2003 with the MS email server setup, the system admin (with the proper permissions) can go in and read your emails with notepad. Same goes for Linux email servers. Maybe Gmail (or another huge service provider) uses a database on the backend, but as far as I know email is just directories and text files.
Even if it does write to a database, assuming that you can programmatically access it, you could build something that works with apache (or runs its own webserver) to translate URLs into doing database lookups.
If you were to actually go about doing this (regardless of whether it is text files or a database), you'd also want to automatically include the URLs within the emails themselves, which I would think many mail servers would let you do.
