DNS is one of those things that usually work everywhere, anywhere, even if there’s a firewall or a security solution in place.
An interesting article to read about this can be found here:
Mail delivery problems to Hotmail, iCloud or Outlook?
We’ll check your server, your DNS records configuration, if your IP/server is blacklisted somehow (and, most importantly, why) and we’ll issue a report or chat with your IT department/support. After that, we can fix all the problems for you or show your IT department/service how to do it.