I'm looking for a way to block or quarantine emails sent from external senders to more than 10 internal recipients in Microsoft 365 or Exchange Online. We typically only deal with 2 or 3 external senders that are legitimate bulk emailers, so we can add them as exceptions. However, I'm having trouble finding a way to set a condition for the recipient count in the Exchange Admin Center. Is there a way to do this either through a hidden option, via PowerShell, or is it no longer supported in the current transport rules?
2 Answers
As far as I know, there isn't a direct way to set that up in the Exchange Admin Center. You might want to check the anti-spam policies instead. You can navigate through security.microsoft.com and create an inbound anti-spam policy to set a bulk email threshold. This might give you the control you need without blocking legitimate emails.
Instead of using transport rules, it's better to set this up using Outbound Spam Protection Policies. They are designed for these scenarios. You can configure them according to this guide: [link]. Transport rules can often get complicated and aren't the best approach for handling mass emails.
Absolutely, using the Outbound Spam Protection Policies is the way to go. It allows for configurations that are clearer and safer than using transport rules.