Can't receive mails. Sender gets "reason: 550 relay not permitted! "

Can't receive mails. Sender gets "reason: 550 relay not permitted! "

I don't know why my email account does not work.
My primary address is with my own, personal domain. (not zoho.com)
The MX settings are 

mx.zoho.com
10
3600

 mx2.zoho.com
20
3600

 mx3.zoho.com
50
3600


and I have set a CNAME  zmverify.zoho.com


What can be wrong?

Gisela