A hostname may not contain other characters, such as the underscore character (_). Doing so may cause the following error when attempting send an email:
Error: The following error(s) must be corrected before continuing The Site URL http://Child_Domain.Domain.com is not valid for sending emails. Please specify a fully qualified domain name for the Site URL within Site and Settings
Remove the underscore from the domain name(_). You can't send emails from NetCommunity when the host name contains special characters.