When sending an email between two domains from different organizations, but belonging to the same On Prem Server, is coming NDR with the error "Remote Server returned '554 5.4.14 Hop count exceeded - possible mail loop ATTR1". The sender uses a mailbox on their Exchange on Premise Server, the recipient uses an Exchange Online Mailbox incl. Exchange Hybrid on their on Premise Server. Emails that do not come from their Exchange On Premise (e.g. from a completely external business partner) are delivered correctly. Why is this behavior occurring? All domains are set up as "Authoritative".
Changed a domain to Internal relay, but the issue exists.