The reason this is occurring is due to a disclaimer rule causing the DKIM to go out of alignment as the messages passes through twice. Once out to ARS and then the second time when it is sent to the internet.
対策
To resolve this issue, the disclaimer rule needs to be moved to the top of the rule list and an exception added to the rule. The exception is : senders address domain portion belongs to any of these domains:
The domain list should include the SMTP domain(s) that the email will be rewritten to: as configured in Domain Rewrite for final delivery to external Internet recipient(s).