There may be an authentication when you try to send but not when you receive.
Fix messages that aren't authenticated
If you get a message that your emails are not authenticated, you should:
Unauthenticated messages and spam classification
Because spammers can also authenticate emails, authentication by itself isn't enough to guarantee that your messages can be delivered, but Unauthenticated messages are very likely to be rejected or classified as spam.
How to monitor
You can monitor the authentication: