This source is known to forward email
forwarding notes:DKIM signatures can be preserved with hosted Proofpoint.
DMARC compliance via SPF and DKIM for customers using Proofpoint hosted for outbound email.
This source can be configured to send DKIM aligned email. Our research has determined that they have gated the related 'how to' documentation. As a result, we recommend opening a support request to get assistance enabling DKIM signing.