a@b is also a valid email address. It is also a valid SMTP address, assuming you have defined "b" as a FQDN in your DNS. What we have here is a list of TLDs w

a@b is also a valid email address. It is also a valid SMTP address, assuming you have defined "b" as a FQDN in your DNS. What we have here is a list of TLDs which have MX records, but an MX record is not even required to send mail to a FQDN. An A record will suffice, for any RFC compliant SMTP server.