Exchange limit the number of internal recipients for external emails

[dmarquesgn]

Hi,

We've had some issues with some external users (like gmail, etc) sending malicious email to a lot of internal users, like 200 or 300 internal users.

So we would like to create some kind of Transport Rule to detect when an email comes from an external domain and if has for example more than 50 recipients, to send to approval. 
But I can't seem to find a way to do it on the Exchange console. Anyone has found out a way to do something similar?
Thanks

[#2 · Accepted solution]

No way to achieve this via mail flow rules, or any other method in Exchange Online, afaik.