Outlook Customer Manager is accessable through Outlook Web Access on Safari. You may receive a message saying "browser does not support caching". To enable caching on safari, please refer to Safari for Mac: Manage cookies and website data using Safari and select "Always allow" option.
What about Google Chrome?
Is there a way to only allow when on a Microsoft website? Also, I don't see the setting that you're referring to in Safari 11.1. I hope it's not one of the first two selections...