Fully Qualified Domain Name
FQDN refers to a host name of a machine connected to the Internet that is typically three or more labels in length, such as "smtp.example.com". A host name may also be a domain name, such as "example.com" or "example.co.uk". While not strictly a FQDN, an IP-domain literal, which is an IP address between square brackets, '[' and ']', for example "[192.0.2.9]" and can often be used in place of host name.One place a FQDN is important and source of many mail delivery problems is with the SMTP EHLO and HELO commands, which both expect a valid FQDN. The argument passed is supposed to be the public host name of the client MTA, which has a valid A/AAA record and should have a valid PTR record, though the latter is not always possible in some cases. With that in mind, outbound mail servers should NOT present internal host names with invalid TLDs to the world, ie. .lan, .local, .localhost, .internal, .corp, .office, etc. as some aggressive anti-spam filtering may reject mail from hosts with bogus names.
- Wikipedia FQDN