Thanks Konstantin
I am using below method to get the User name and then follow the normal approach (usr21, adr6) to get the email address.
CL_BBP_ES_ENTERPRISE=>GET_EMPLOYEE_BY_GUID( GUID )->IF_BBP_ES_PERSON~GET_USER_NAME()
and for GUID, I am using Partner Number.
Just confused as I am new to SRM. As the method says "Get employee by guid" , so it means all the Buy on Behalf users are registered as employee ? just want to know if there is any exception.
Thanks
Nishant