explicit proxy - user UPN in AD does not match our domain name and thus LDAP lookup fails

I have an issue with explicit proxy authentication with NTLM with LDAP lookup to AD - in summary our user UPN in AD does not match our domain name and thus LDAP lookup fails Our domain is exampleAD.com My user is user1@exampleAD.com My UPN is user1@example.com If I change my UPN back to user1@exampleAD.com the user authentication in explicit proxy then works, but we have a requirement (Office 365) for our UPN to match our email domain name, thus must remain different to domain user name. Any ideas what we can do to resolve?

 

Thanks in advance

 

 

The log details if it helps are below - but not sure this is relevant as I know the workaround is to change the UPN.

 

GeneralDate2019/01/21Time11:25:51Virtual DomainrootLog DescriptionExplicit proxy user group query failed

SourceIP10.10.10.10Useruser1@exampleAD.comGroupN/A

DestinationIP212.58.249.208Host Namewww.bbc.co.uk

ActionActionNTLM-authPolicy0StatusfailureReasonGroup information query failedAuthentication ProtocolHTTP(10.10.10.10)

SecurityLevel 

EventMessageUser failed in group information query