https://support.microsoft.com/en-us/topic/smtp-connection-fails-when-you-log-on-with-a-child-domain-account-and-use-ntlm-authentication-in-exchange-server-2013-91281eef-aad2-7c58-8ab3-92fba00ffe7f
Fixes an issue in which the NTLM authentication fails in an SMTP session when the UserPrincipalName property of the authenticating user references a child...
smtpconnectionfailslogchild