Hi,
Thank you for your questions in the forum!
Based on your description, I understand that you want to receive emails that are treated as external emails from services without changing your security policy.
The following is the fastest and easiest way I have personally come up with. It is recommended that in Outlook you create a new mailbox specifically to act as a relay for forwarding emails from external service. When DL needs to receive external emails, first send them to this new mailbox and then forward them to the group list.
I am trying other solutions and will keep you updated if there is a better way.