Question : Problem: Off-site backup for Customers with web interface

We want to offer off-site backup for our customer's for their email and other files.
Does anyone know of any Windows based software that would run on our servers, and then a customer could login with their username and password and search through their archived emails/files?

I'll take any Windows & open source solutions.

Answer : Problem: Off-site backup for Customers with web interface

If your goal is to get your clients data from their servers across a WAN to your local servers where you then back the data up to tape then  have a look at Doubletake.
Just install the Doubletake software on each remote server that you want  to backup.
At your end you have a server with Doubletake software and a tape drive.
You can sync many remote servers to your one local server.
You can even offer DR capability to your clients using Doubletake High Availability failover.
The software only syncs the the data that has changed and works at the byte level.
www.doubletake.com
Cool product, we use it to sync to our DR center.