I would suggest adding the sender and or domain to your organization allowed senders policy and enable allow to bypass attachment policies as long as this is a trusted sender.
The recipient of this message can contact the team responsible for their support to request a false positive investigation. This will help determine why the message was blocked, and most likely have the filters adjusted to allow the messages going forward. An allow list entry will also allow the message from the sender to be delivered if the customer does not wish to open an investigation.
SaaS Email and Web Security
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