My Free Zoho account is bouncing back some, but not all, emails.

My Free Zoho account is bouncing back some, but not all, emails.

When someone emails me, they receive this message 

Delivery to the following recipient failed permanently:

     pippa@sundayassembly.com

Technical details of permanent failure:  
The recipient server did not accept our requests to connect. Learn more at   http://support.google.com/mail/bin/answer.py?answer=7720  
[(0)   sundayassembly.com . [50.17.211.181]:25: Connection refused]

But some emails are getting through. Can't see a pattern as to which ones get through and which don't. Have tried deleting a lot of the inbox, hasn't made a difference. 

Please help! I am in the middle of a tour and so contact is real important.

Best

Pippa