421 4.4.0 Remote server response was not RFC conformant'

Copper Contributor



i have a problem when send a mail to this domain sifa.it, i receive the error message in subject from my exchange server 2016. I have alredy check the dns and the ptr record and all work fine.

any suggestion?


1 Reply

Hi @Massimo120 


This would indicate that the problem is with the server you are trying to send the email to (the remote server) - Exchange is saying that it reached the email server, but the response it got wasn't what it expected so can't send email. You might need to try an reach out to the IT team who look after the remote server for assistance.