I am facing the same error code 554.5.1.8.

Via the control panel I tried to change the SMTP block - failed.

Also tried to unblock the email address - failed 

All lead to the message I should contact  support@zoho.com - done 

Now I am worried because my business depends on it.

Question:

- How to avoid this - it happend without any notice or warning .. 

- What can I do to solve this 

Thank you

Best Regards

jan@valocom.nl      (blocked SMTP - outgoing mails) .... :(