When this option is set to Yes, Virtualmin will automatically attempt to create a hidden host default domain (virtual server) with a Let's Encrypt certificate during the next configuration check. If the Let's Encrypt certificate cannot be successfully requested for the system hostname, the host default domain will be automatically removed.

However, it is possible to use Yes, and keep visible, option to keep host default domain visible, and not have it removed in case Let's Encrypt certificate cannot be requested successfully, which will allow for manual adjustments later on.

If the system's hostname changes, Virtualmin will automatically try to update host default domain name accordingly on the next configuration check.

Host default domain can be disabled by setting this option to No, which will make it to be removed from the system on the next configuration check.

Be advised that using host default domain for mail is highly discouraged, and may cause problems with mail delivery!