There's no way to do this natively, you will have to develop some kind of add-in. DLP is a different use case and not applicable here, and the other functionality that comes close to this is MailTips: https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/mailtips/mailtips
Neither of these will be a good match for your scenario though, thus my recommendation is to look into custom solution.
Outlook App / Outlook Web custom infobar messege

Oskar Pawłowski
1
Reputation point
Hello,
I would like to educate users that they should use third-party document management storage instead of e-mail's attachments. So I looking a way to display a messege in Outlook's inforbar (like mail tips or policy tips ) for specific conditions:
- all receipients are from my organization (domain)
- a file is attached
Could you advice me how can I do it? Data Loss Prevention don't allow to filter external recepients from custom rule.
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Vasil Michev 66,601 Reputation points MVP
2020-12-11T15:01:32.713+00:00