smtp 550 error. Mailbox unavailable

smtp 550 error. Mailbox unavailable

Hello,
I seem to be unable to forward or send messages to email addresses at my zoho mail. I get the error:

Final-Recipient: rfc822; t.vlachopanagiotis@rhoe.gr
Action: failed
Status: 5.0.0
Remote-MTA: dns; mail.rhoe.gr. (185.4.133.16, the server for the domain rhoe.gr.)
Diagnostic-Code: smtp; 550 Requested action not taken: mailbox unavailable or not local.
Last-Attempt-Date: Mon, 30 Apr 2018 00:45:40 -0700 (PDT)

Any ideas what might be the cause of it?
Thank you