Mailbox name not allowed. The server response was: Relaying disallowed

Mailbox name not allowed. The server response was: Relaying disallowed

I am trying to send email to my zoho account, to my custom domain (not zoho.com) from another email account ie. gmail.com.
Every mail i send i get the following error:

mx.zohomail.com #553 Relaying disallowed ##


What can i do?

Thanks!