Please read my blog's comment policy here.
Internet Explorer (actually, WinINET, the network stack beneath IE) prohibits use of certain ports for HTTP(S) connections. The intent of this blocking is to prevent Cross Service/Protocol Request Forgery attacks. For instance, an attacker could use a HTML form to send a request to an unprotected mail server such that the mail server interprets the request as a valid (albeit poorly-formatted) request to send an email message. Such attacks are obviously interesting to spammers and other bad guys.
IE8's current port-block list contains:
19 (chargen), 21 (ftp), 25 (smtp), 110 (pop3), 119 (nntp), 143 (imap2), 220 (imap3), 993 (secure imap)
Blocking ports 220 and 993 is new to IE8.
Attempts to use these ports in HTTP/HTTPS URLs will result in a connection failure. At this time, WinINET does not offer users or administrators a mechanism to block additional ports or unblock ports.
Other browsers attempt to block other ports; Firefox, for instance, blocks a larger set of ports by default.