Section 2.1 of the discover document includes a port number on the authority based on the URI RFC (see below). Unfortunately this is not consistent as no scheme in wide use allows this behavior, and AFAICT there was no standard scheme in place that allowed it at the time RFC 3936 was written. In general a URI MUST have a scheme. The real problem is that since the scheme mailto: does not allow a port number, it is not practically possible for a user at a computer to enter a user name that contains a port number. I strongly recommend that port numbers in authorities be removed since it is not possible in practice to discover the port number.
NOTE: Since the definition of authority in RFC 3986 [RFC3986] is [ userinfo "@" ] host [ ":" port ], it is legal to have a user input identifier like userinfo@host:port, e.g., email@example.com:8080.