Message delivery to Google / gmail fails with the following message in the Message Audit Log delivery details:
550-5.7.1 [IP ADDRESS] messages missing a valid address in from: header, or 550 5.7.1 having no from: header, are not accepted. - gsmtp
The message has a From header which either does not contain an email address or contains a malformed email address.
The sender email address in the Status > Message Audit Logs is not the same as the From: header in the email message. To view the From header
Once you know how the message is being improperly constructed, you can address the issue with the sender or application server which is generating the messages with missng or malformed addresses in the From header.