553 Relaying Disallowed error when using MX Guarddog Spam Services

553 Relaying Disallowed error when using MX Guarddog Spam Services

To cut down on spam I route all my mail through the MX Guarddog service. My DNS MX records therefore point to the MXGD servers and NOT the Zoho mail servers.

I then setup In MXGD, I then point the system to the ZOHO mail servers at mx.zoho.com port 25. I've also setup a secondary server as mentioned in this forum post https://help.zoho.com/portal/en/community/topic/mxguarddog 

Doing a test I get a 553 Relaying Disallowed error. I also get this from a gmail account.

I've added the MXGD IP addresses into the Incoming Gateway settings on ZOHO but still no difference and not sure what to do next.

Domain is largo.co.uk.

Please help!