Check the receiving server's IP address and PTR record to make sure that it is authorized to relay mail.

Check for any block lists that may be providing a blacklisting or other preventing mail from being transmitted or received.

Steps to Resolve Issue:

1. Verify that the server’s IP address is up to date.
2. Check for any block lists that may be blacklisting the domain.
3. Check for any problems with the firewall that may be preventing outgoing mail.
4. Verify that any necessary encryption settings are correct.
5. Verify that the domain’s DNS records are up-to-date.