outside the organization message

This is heavily requested by several clients, since Exchange already does this.

The client wants a “whitelist” of domains that belong to their organization or groups of company,

any email NOT belonging to that list should have a custom message (at the top) that indicates outside email, this helps a lot with domain phising attacks (where the attackers changes 1 letter of the domain making the user fall into trap) and other similar phishing incidents.

Right now in zimbra blog there’s a way to custom code this, but it doesnt work properly, because if the email comes from m365 or google, the custom message gets deleted.
we need a way to implement this in zimbra at the admin level. Or an updated tutorial with a way that this would work properly
Thanks!