When Exchange Online users attempt to send an email, the remote Server returned 550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP.
This problem usually occurs with users who are in a trial period and only with emails sent from Outlook.
You must perform the Release Tenant IP Not Accepting Traffic procedure. However only Office 365 support can do it. Therefore, it is necessary to open a ticket to solve the problem.
To open a support solution, you must have access to the “Admin center” of the tenant:
Now the NDR 550 5.7.708 should no longer be returned.