A Fully Qualified Domain Name (FQDN) is the complete domain name for a specific location in the Domain Name System (DNS) hierarchy. This is sometimes referred to as the absolute domain name.
The hierarchy can be broken down into a few parts:
The format of FQDN includes all hierarchical parts, usually [hostname].[domainname].[tld]. A mail server, for example with SendGrid, could be specified by an FQDN such as
mx.sendgrid.net as this provides the complete domain for a specific mail server. When connecting to a host the specification of a FQDN allows for the DNS to resolve the hostname to its specific IP in order to verify the connection and successfully send mail.