Hi @Glenn Maxwell ,
Do you want to enable MailTips for your users? (Outlook for windows and Mac users see the message "The following recipient is outside your organization" when sending emails to external users.) If I have misunderstood you, I apologize and ask you to correct me.
The MailTip informs the sender that the message will leave the organization. By default, this MailTip is turned off. You can turn it on using the Set-OrganizationConfig cmdlet.
You could refer to MailTips in Exchange Online
To enable MailTips in outlook, you could refer to: Outlook MailTips Options
In the new version of Outlook for Mac, you could also enable Mail Tips: The new Outlook for Mac
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