Ask those users who the vendor of the firewall is (or have them ask their IT department) and reach out to them. It's probably one specific vendor. It might not

Ask those users who the vendor of the firewall is (or have them ask their IT department) and reach out to them. It's probably one specific vendor. It might not even be on any kind of list, could easily be some kind of heuristic matching some traffic pattern or even it not liking your domain name or IP somehow. You can check your domain in things like https://talosintelligence.com/reputation_center/lookup?search=crewsum.com https://talosintelligence.com/reputation_center/lookup?searc... or other domain reputation engines. But nothing seems wrong with it.