Let's Encrypt error getting new cert

I installed letsencrypt, specified the path, and tried to generate a new cert for my virtualmin installation, and I get the following error:

Checking for new version... Requesting root privileges to run letsencrypt... /root/.local/share/letsencrypt/bin/letsencrypt --no-self-upgrade certonly -a webroot -d xxxx.xxxxxx.com --webroot-path /home/xxxx/public_html --duplicate --config /tmp/.webmin/999801_56362_1_letsencrypt.cgi An unexpected error occurred: The provided email for a registration was invalid :: DNS problem: NXDOMAIN looking up MX for array(0x4ac4f10) Please see the logfiles in /var/log/letsencrypt for more details.

The logs don't provide any more helpful info. Any idea what's going wrong?

Thanks, Don