Problem

Your standard inbound mail configuration for alumni accounts includes an option that modifies the message body to contain a warning message in the form of a banner, colored text, etc..  to alert the recipient that it was sent from an external source outside their organization or network.  This breaks the DKIM signing by the original mail system.   When the message is finally delivered through the Alumni Forwarding system, the receiving mail system classifies it as Spam.

Example of a typical 'External' banner that can be added to a message body by M365:

Additional Information

Mail systems like Google will perform SPF/DKIM/DMARC checks and may classify a message as Spam based on them.  If the initial system which receives emails to alumni from an external source makes any changes to the body of the message, that will break the original DKIM signature.  Despite the usage of ARC, this doesn't mean that all receivers will trust the changes made to the message by M365.  A failed original DKIM signature can weigh significantly within the Spam scoring of a message.

Recommended Solution

Do not make any changes to the message body betwen when it is received by your mail server and when it is relayed to the Alumni Forwarding service.  In M365, this may require some re-configuration of the Mail Flow Rules and / or other configurations in M365 which can be used to apply the 'External' message.