My server is a SMTP smart host which diabled open relay. I have lots of bad mail in my c:\inetpub\mailroot\badmail (550 thousands bad mails in 2 months). the event log will generate event ID 4000
I open the bad mails and found that most of them are non delivery reply mail from my postmaster. I have disable NDR
Message delivery to the remote domain 'boac.com.au' failed. The error message is 'An SMTP protocol error occurred.
'. The SMTP verb which caused the error is 'RCPT'. The response from the remote server is '550 5.1.1 <email@example.com> is not a valid mailbox
Message delivery to the remote domain 'rrk.nu' failed for the following reason: The remote server did not respond to a connection attempt.
Message delivery to the remote domain 'drillerssupply.com' failed for the following reason: Unable to bind to the destination server in DNS.