It's frustrating when you get an error after sending an email message. This topic describes what you can do if you see error code 550 5.7.64 in a non-delivery report (also known as an NDR, bounce message, delivery status notification, or DSN).
Only an email admin in your Microsoft 365 or Office 365 organization can fix this issue. Contact your email admin and refer them to this information so they can try to resolve the issue for you.
I'm an email admin. How do I fix this?
This problem happens when you use an inbound connector to receive messages from your on-premises email environment, and something has changed in your on-premises environment that makes the inbound connector's configuration incorrect. For example:
The host name that's specified in the certificate on the inbound connector no longer matches the source email server.
IP address of the source email server no longer matches the source IP address on the inbound connector.
The Diagnostic information for administrators section in the bounce message will contain the original error message when Microsoft 365 or Office 365 tried to send the message to the external email server or service.
This module examines how Exchange Online Protection (EOP) protects organizations from phishing and spoofing. It also explores how EOP blocks spam, bulk email, and malware before they arrive in users’ mailboxes.
If you’re an administrator who deploys and manages Microsoft 365 and performs Microsoft 365 tenant-level implementation and administration of cloud and hybrid environments, this certification is designed for you.