URL and ports for CrossRef XML Export Plugin

Hello, I hope all are doing well. We are on OJS 3.3.0.14 and trying to use CrossRef XML Export Plugin and getting errors about not being able to contact the server. We have a very restrictive firewall in place. Here is my question what URL’s and ports to the CrossRef XML Export Plugin need to communicate with CrossRef? If somebody know the servers or servers then we can have them added to the firewall exception list. Thanks in advance!

Sincerely,

Eric T. Gadsby