CheckUserExists()
From Rackspace Email & Apps API
This function will check to see if a mailbox or alias name exists on a particular domain.
Input Parameters
Four strings:
Administrative username, Administrative password, Domain name, Mailbox/alias name to check
Return Values
If the user exists, the return value of the function will be 1. Otherwise, the function will return 0.
Consult the Result Code Table for a list of result codes and their meaning.