We have had for years a supported Non TLS connector, filtered by fixed public IP Address on O365 that we use to relay email for our printers and monitoring tools. This is set up on a tenant we have for years and It works flawlessly but due to having sold part of our business we need to change this configuration. I thought, "not a problem, let's get a new tenant on a different domain, set it up the same way and it will work". Wrong! Didn't work, supposedly because now Microsoft by default enables Security Defaults on new tenants (Annoying MFA, Etc), sure you can disabled it but still doesn't work at least immediately and support seems to have checked everything I did and reached no brilliant solution.... On why one Tenant works and other won't which is worrying! Just one idea of having something to do with tenant dehydration... They will call me today again to follow this, I'll update this thread afterwards. If someone has any thoughts they will be more then welcome.