Originally posted by: GeneralDisarray
They can easily specify an alternate port in the URL, and if you blocked that one, they could specify a random port and set their web server to listen on all ports.
As some have said above, it's quite simple to GET variables in an image link to identify when someone views your message. Theres no need for different file names. In fact, you dont even need to have a link that looks anything like an image, you can manipulate the MIME headers so something like <a href="http://127.0.0.1/?go=123456">http://127.0.0.1/?go=123456</a> returns an image and IE/Outlook loads it fine. Then in their server a PHP script checks the ID and logs when it was accessed, and if they're smart, watches if you click on the link to see if the advertising was effective.
okay, so only grant access to smtp traffic on port 25 (or whatever port it is on).
