Hello
We have the same error in our environment (Exchange 2013 OnPrem).
In logs path: C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpSend
are we can find the error:
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,142,SERVERIP1:29450,SERVERIP2:2525,>,MAIL FROM:<sender@XXXXXX.COM> SIZE=0 AUTH=<> ENVID=005056937B6C1ED8A181EEC6F088C143,
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,143,SERVERIP1:29450,SERVERIP2:2525,>,RCPT TO:<rec@XXXXXX.COM> ORCPT=rfc822;rec@XXXXXX.COM NOTIFY=FAILURE,
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,144,SERVERIP1:29450,SERVERIP2:2525,<,250 2.1.0 Sender OK,
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,145,SERVERIP1:29450,SERVERIP2:2525,<,250 2.1.5 Recipient OK,
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,146,SERVERIP1:29450,SERVERIP2:2525,>,DATA,
2018-07-10T07:04:19.404Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,147,SERVERIP1:29450,SERVERIP2:2525,<,354 Start mail input; end with <CRLF>.<CRLF>,
2018-07-10T07:06:23.102Z,Inbound Proxy Internal Send Connector,08D5DD0D5D6BB6FD,148,SERVERIP1:29450,SERVERIP2:2525,*,,Proxy
layer started discarding data. Acking message as failed.
2018-07-10T07:06:23.102Z,Inbound
Proxy Internal SendConnector,08D5DD0D5D6BB6FD,149,SERVERIP1:29450,SERVERIP2:2525,-,,Local
Every day we can find some discarded messages and we are not be able to trace the message with the command get-messagetrackinglog. It seems that the message is discarded and deleted and we can't find trace of it...