No doubt you send e-mails containing hyperlinks that link to an external website. Be that a squeeze/landing page, your homepage, or any other external website like Google. One thing most people don't know is that using URL's as display text for these links can cause delivery (and click through rate) issues.
Within your account, there is a feature pre-added to track each of your links sent out in an email. The system automatically assigns each link a unique tracking URL. So, when your contacts click on a link, they are temporarily directed to the tracking domain (clk2view.net/uniqueidhere), after they hit that tracking URL, the click is logged, and the contact is then immediately directed to the proper web page.
If using a URL as display text, most ISP's will see the URL you are showing people and notice that the URL they are immediately being directed to is different, thus they think your trying to trick the customer into visiting a website they don't want to visit, or perhaps that you're posing as someone you're not.
For example, I could send you a link like this google.com, to the untrained eye it would look like clicking on this link would cause www.google.com to load up. However, that is not the case (go ahead, click the link). By using google.com as anchor text, I could then send you to any page I wanted, or even pose as Google and request credit card information.
Obviously, ISP's do not want their customers falling victim to such scams and have added phishing filters to account (phishing is the act of deception I just described) so they typically either block e-mails that have hyperlinks with URL's as display text or put a large warning on them that says something like "This e-mail is trying to trick you. We don't recommend clicking on any of the links" which as can imagine is very detrimental to clicks as well as in box delivery rates.
Please feel free to post any questions or comments on the matter here by replying to this thread!