Email Receiving: SES sends failure notification even if message is delivered

I set up email receiving. My domain is verified.

I have an active rule set, consisting of one rule, matching my entire domain.

The rule has only two actions:

1. Deliver to Amazon S3 bucket (with SNS notification)
2. Stop rule set.

Spam and virus scanning is enabled.

Everything is fine: recipients receive messages.

But if the status of the dkimVerdict and/or the dmarcVerdict is GRAY for a received message, the sender receives a failure notification, as if the message was rejected, even if it's successfully delivered.

The diagnostic code in the notification is smtp; 451 4.3.0 This message could not be delivered due to a recipient error. Please try again later..

I tried to send messages from domains with DMARC policies correctly set (both verdict show status PASS) and I didn't get failure notifications.

Anyone else facing this issue?

Thank you! Paolo