you have to enable connecotor logs with verbose log level!

1. find session id on receive in
   C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive
   search recipient or sender….you will find also the message id!!!
2. show all lines of this session
   @(Get-Content -Path „C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive\RECV20150429-1.LOG“) -match „08D24ED360CF3B98“ | Out-GridView
   see connector id, etc.
3. find session id on send to hup
   C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpSend
   search by message id, recipient or sender…
4. search session id in current log file of frontend send connectors
   @(Get-Content -Path „C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpSend\SEND20150429-1.LOG“) -match „08D24ED360CF3B98“ | Out-GridView

   do the same for following files

5. C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Hub\ProtocolLog\SmtpReceive\RECV20150429-1.LOG
6. C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Hub\ProtocolLog\SmtpSend\SEND20150429-1.LOG
7. C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Mailbox\ProtocolLog\SmtpReceive\Delivery\RECV20150429-1.LOG