Mail server error

Mail server error

On 16 december I received last email message.
After that my email TheBat client did't receive any new email.
| checked Log file and found today: 
18.12.2018., 11:20:35: FETCH - Server reports error. The response is: -ERR Service Unavailable

When I sending from another email to zoho, I received error:

The mail system
< radenko@zoho.com>: host smtpin.zoho.com[8.40.222.120] said: 552 5.1.1
    < radenko@zoho.com> Mailbox delivery failure policy error (in reply to RCPT
    TO command)

What to do ??

Please send me answer to radenko@ni.ac.rs 

B.Regards