The route domain path is incorrect. This produces a "Host unreachable" error.

Check the path on the General tab of the Domain Properties for the remote domain.

The receiving server is down or the message cannot be delivered for a variety of reasons.

Consider the following possible causes:

• Check the .rtr files in the Queue directory to determine the cause.
  
  
• Check the retry counters in Reliability and Performance Monitor.
  
  
• Check the receiving server.
  
  

The name is incorrect or the address is invalid.

Check the name and path.

Transport Layer Security (TLS) is required for connections. The key certificate is invalid or the encryption levels don't match.

Consider the following possible causes:

• Check whether the use of TLS has been overridden on the Outbound Security settings in the Domain Properties for the remote domain.
  
  
• Check the TLS encryption level that is used by the SMTP service certificate. Make sure that it matches the level required by the receiving server.