Hello,
Yes, in backup exec 9.1, there was a different license n umber for Exchange 5.5 and a different one for Exchange 2000 and 2003. Refer to the link below that explains on how to find this difference:
How to differentiate between Backup Exec Agent for Exchange 5.5, Exchange 2000, and Exchange 2003 in Backup Exec for Windows Servers 8.x, 9.x, and 10.x
http://support.veritas.com/docs/253590
You may get in touch with your reseller or our sales team at:
http://www.symantec.com/corporate/corpdir.html