Error 11002 is WSATRY_AGAIN, which means "Nonauthoritative host not found" . It's an issue at the TCP/IP layer and its documentation states:
This is usually a temporary error during host name resolution and
means that the local server did not receive a response from an
authoritative server. A retry at some time later may be successful.
In any case, this seems to be caused by some kind of connectivity issue unrelated to your application and Rebex library.
It might be useful to use
LogWriter property to create a communication and error log that shows what has been going on inside Rebex SMTP. Feel free to post the log here or mail it to firstname.lastname@example.org for analysis.