Authorize.net connection problem

I'm working on setting up a new store for a client and when testing I get this system error:

"The connection to the payment plateform did not succeed. It is often caused by the hosting company blocking external connections so you should contact him for further guidance."

I've talked to my host (Rackspace) and they said that port 443 is open, they aren't see any errors in their log when I test it, and they don't see any other issues would be be preventing this from connecting. What else could be the issue here?

It could potentially be that you changed the URL option of the authorize.net plugin to something else which is not correct.
Other than that, I don't see anything else that could produce such problem.

What should it be set to when using AIM?

That's what I was missing. Thank you!