When your UserPrincipalName (UPN) property references a child domain of the forest's fully qualified domain name (FQDN), the NT LAN Manager (NTLM) authentication fails in a Simple Mail Transfer Protocol (SMTP) session in a Microsoft Exchange Server 2013 environment.
This issue occurs because of a bug in the code that causes the UPN of the user in a child domain of the forest to not match the FQDN of the forest during the NTLM authentication process.
To work around this issue, change the UPN of the user to match the FQDN of the forest.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Article ID: 3056413 - Last Review: Jun 16, 2015 - Revision: 1
Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard Edition