Is anyone able to contact GTT?

I sent an email to noc at gtt.net from 2 different emails and both got a reply saying: 5.1.0 - Unknown address error 550-'5.4.1 Recipient address rejected: Access denied [HE1EUR01FT058.eop-EUR01.prod.protection.outlook.com]' Not sure if this means if they are blocking my email or if their email is broken.