Using Exchange server 2003 on premises server connected to Exchange online protection server for spam filtering. Sometimes when receiving email from the Exchange online Protection I get SMTP error 451 4.4.0 SMTPSEND supiciousremoteservererror remote server disconnected abruptly on the Exchange online Protection server. The exchange server will receive the email and process it to the user. The problem is the Exchange online protection does not think it sent the email so it sets the email as pending and will try to send it again several times. This causes the user to get duplicates of the email. I have opened a case with Microsoft and they said everything was correct with the exchange online protection server. Any one ran into this problem? It is on all cisco network and I have checked that smtpfixup is disabled.