M365 has resolved the issue and notified the small set of users that were affected.
Please note that if you have received a bounce back error “503 5.5.1 bad sequence of commands” this means your email has not been sent. And we suggest trying again in about 30min or when we get confirmation from Microsoft that the issue has been resolved.
We are closely monitoring the situation and will keep you promptly updated on Microsoft’s status.
Microsoft has found an issue this morning which is causing some users sending emails to receive a bounce back.
This bounce back will state the error “503 5.5.1 bad sequence of commands”.
This is currently being investigated by Microsoft, we will let you know of any critical developments on this matter.