You’ve recently turned off ‘Open Relay’ as it caused blacklisting of your IP due to unauthorized use.
Now incoming emails are being rejected by the server.
Telnet (or Netcat) to the server to confirm the issue. It might show an error
554 5.7.1 Access
Check the SMTP service settings and disable the ‘Local Clients’ IP filter as this might be blocking external connections.
This might have been enabled to avoid abuse of the server while resolving the ‘Open Relay’ configuration.