Hello Sir,
Well, when creating new account, we are checking in background if such account already exist in specific portal, depending on the region (there are 3 portals: NAM, EU, APJC). Withing the region user much have a unique account for each organization.
If you wish to use the same email address for multiple customers I could suggest to use following format:
user+customer1@cisco.com
user+customer2@cisco.com
user+customer3@cisco.com
etc.
When email is sent, everything between the + and the @ will be ignored.
All of those will end up with emailing at user@cisco.com.
Hope that helps.
Wojciech