Sometimes, when a mailbox has been up to no good, started making trouble in the neighborhood, the automated security systems within the 365 ecosystem will assign a reputation and apply corrective action to that mailbox. This most commonly appears as a mailbox recieving mail, but unable to send, with sent mail returning a system mailer daemon error message.
The fix is very simple. On Portal.Office.Com, navigate to the Security Admin Portal > Email & Collaboration > Review > Restricted Entities.
Find your mailbox in the list and unrestrict them.
Done.