It may be that filling out their stupid form makes it work for a few days, but then it comes right back.
550 5.7.606 Access denied, banned sending IP [184.108.40.206]. To request removal from this list please visit https://sender.office.com/ and follow the directions. For more information please go to http://go.microsoft.com/fwlink/?LinkID=526655 (AS16012609)
I guess this is any recipient who has the poor taste to be using Outlook for their email, in this, the year 2019?