After someone who received one of our eTap mass emails let me know that a link was broken, I contacted support, as that same link had been working in the test email I sent to my own email address. I was informed by support that the test feature available when drafting a mass email does not allow for accurate testing of links:

Yes, the test doesn't actually go through the advanced mass email server - just an eTap server. So it doesn't go through the same translation as far as trackable links go.  The best thing to do rather than use the Test is to create a Custom Account Query of just your account, and actually send the Adv. Mass Email to that query (it only contains your account). Then it will be exactly as it is during the actual sending process.

As I'm sure many users do, I use the test to ensure that links are working.  Customers should be informed that this feature does not work for this purpose.