I have an issue sending workflow email, to an email enabled AD security group. I sandwiched the AD group inside a SharePoint group and give permission in the site(this is the usual way to send email to individuals within the AD group). Why the people in the AD couldn't receive the email( I also added my self as individual and able to gate the email)
It could be a restrictions in exchange server which prevents SharePoint to send an email to this AD Security group, need to check the Delivery management for this group and allow SharePoint to send messages to it.
I can reproduce your issue in SharePoint Online.
It’s by default that the members of security group cannot receive the email.
We need to create the Mail-enabled security group and add the users to it.