We have two, external to our tenant, email address that are getting rejected as:
550 5.1.8 Access denied, bad outbound sender AS(41000001) [BN8NAM04FT064.eop-NAM04.prod.protection.outlook.com]
I have seen this rejection internally when someone sends too much spam or an account is compromised, they need to be removed from the Restricted Users bucket. This is only internal people though, not external ones.
I'm at a loss, I've had a ticket open for at least a month with our free support and it has gone nowhere.
Just curious if anyone else has run into this and if they found a solution.