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 resolve this issue, install Cumulative Update 9 for Exchange Server 2013.
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.