Hello @Pennoni ,
Most email security systems, especially for government, company, and other large organizations, prefers authentication from one server as a means of mitigating spam and avoiding malware - even if the ESP being sent from is trusted. You can only have one domain authenticated per account. Unfortunately, this is just how emails work for these larger entities nowadays, regardless of which ESP is used.
If you need more info on self-authentication, and the processes involved
Deliverability's numbers and hours if you need more specialized guidance