One of the most common design considerations for archiving is where to store the data.

This seems like a fairly simply solved issue until you run into issues of scale. Quantity.

Some file systems have definite limits on the number of files you can store, on the total size of the volume(s) and whether or not you can just add to it.

It is vitally important that you overestimate your total storage needs before choosing where to store your data.

If you choose a file system with a hard limit and a year or two down the road, you run out of space ... you might be unable to continue to use your system. You might even have to create a new archive. That diminishes the usefulness of having everything in one place.

When quantities become huge, many people turn to NAS or SAN technologies.

Whatever you choose, be sure it will be adequate for your mandated retention peroid.

I'd like to share this link with you, from Novell's site - a primer on file systems:
http://wiki.novell.com/index.php/File_System_Primer

It's a great overview of the current state of affairs on file systems available today.

I hope this is a useful point in the right direction as you design your own e-mail retention system