Are we allowed to promote external websites, services, etc.?

Are we allowed to promote external websites, services, etc.?

Let's say I wanted to set up a Service where for a tiny fee, I would provide a discount code good toward a hosting account at some web host. Would this be allowed? I tried doing this on F***rr, (after carefully reviewing their TOS first), but got denied due to their "editorial guidelines" that they don't publish or otherwise make available for viewing, which apparently prohibit any mention of external services or websites. For that matter, is there any list of types of Services that are prohibited (aside from obviously illegal ones, for example)?