Hi @Adrian Ducet ,
Welcome to the Microsoft Q&A platform!
Based on your description, it sounds like you have a duplicate accepted domain in your Exchange Online environment, which is causing the issues. This can happen sometimes during domain migrations or configuration changes. To resolve this, please kindly delete one of the duplicate accepted domains to see if it works.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".