A client recently came to me with a problem where they could not login to MS Dynamics CRM via Outlook. I discovered that they had a network firewall enabled so I went about researching which ports are used by CRM for Outlook. This TechNet article shows that port 80 is used for HTTP and port 443 for HTTPS. I went back to my client with this information but it turns out that they don't block either of those ports. We learned that the problem is only with the initial login so they temporarily disabled the firewall and was able to login. Once the firewall was enabled again, it was still working.