We are making two new changes to the way that the email system processes mail in an attempt to cut down on spam. We are adding additional validations and checks to ensure that the domains mentioned in the SMTP envelope are valid and have a functional DNS record.
These changes impact inbound and outbound customers differently, but the core change is the same. If the sender or the recipient of a message is not a registered domain name we will defer accepting the message.
Outbound SMTP: https://support.duocircle.com/solution/articles/5000788780-check-to-see-if-a-recipient-domain-exists
Inbound Gateway: https://support.duocircle.com/solution/articles/5000788778-check-if-the-sender-domain-exists
You will be able to whitelist inbound domains.