Symptom:
Unable to sign in with UPN@ServiceDomain if _cuplogin is discovered

Conditions:
Step to Reproduce:
1. configure _cuplogin in DNS
2. configure userPrincipalName as LDAP Attribute for User ID in Cisco Unified Communication Manager
3, sign in with username@example1.net@example2.com to discover service
4, input correct password in sign-in screen

Expect Result:
1, Jabber should be able to sign into Cisco Unified Communications Manager IM and Presence service

Actual Result:
1, Jabber prompts invalid credential. 
2, only when the username is manually changed to username@example1.net, Jabber can sign into Cisco Unified Communications Manager IM and Presence.