I added a link to an external page in one of my emails, when I receive the email and click on the link, it appears to be broken, but I am sure the URL is correct.
Marketo adds code to links in emails as we send them out to track clicks. If the destination page is one that does not support URL parameters, the page may not load. This is very rare but in the case that it happens in your testing, you can try disabling tracking for that specific link.
See Disable Tracking for an Email Link for detailed instructions.