Erroneous SPAM label incoming, CXSNDR error outgoing

Mail from a certain domain, which previously arrived with no problem,is now going to my SPAM box, and mail from me TO that domain is failing with a CXSNDR error message.  Tech support told me to click the "This is not spam" button above a message in the SPAM box, to stop those messages from being flagged as spam AND also permit me to send to those addresses without the CXSNDR error.
This did not work. Messages from that domain are still blocked, incoming and outgoing. What should I do?