url-bad-port-list

url-bad-port-list is a variable defined in `url-vars.el'.
Its value is

("25" "119" "19")

Documentation:
List of ports to warn the user about connecting to.
Defaults to just the mail, chargen, and NNTP ports so you cannot be
tricked into sending fake mail or forging messages by a malicious HTML
document.

You can customize this variable.