ERROR_CODE :550, ERROR_CODE :permanent failure for one or more recipients ...blocked

ERROR_CODE :550, ERROR_CODE :permanent failure for one or more recipients ...blocked

I do a lot of business with one company, domain craigbachman.com, and as soon as I switched over to Zoho, I have not been able to send emails to anyone there.  They all come back as   ERROR_CODE :550, ERROR_CODE :permanent failure for one or more recipients (....@craigbachman.com:blocked), and it is Zoho blocking it.
I was receiving emails from them at first, but I am no longer receiving emails from them, either. I have tried whitelisting them, but it is not working. This is a HUGE problem for my business!