====== 553 Requested action not taken: mailbox name not allowed ======

Such error is returned to you by email servers in reply to messages sent from Thunderbird, Outlook, or any other means, when the recipient email it typed with illegal characters, maybe because doing copy/paste and some unwanted characthers have been included, or simply because of typos.

e.g.
**not\allowed@example.com**

===== Solution =====
Simply resend the email removing the illegal characters.