There are some Security groups in AD that are set up with email settings that
will work and there are other ones that don’t have an email tied to them that
will not work.
For alerst so to work with AD groups it must have an email address in the email property. If you add one after it's been synced into SharePoint already you may have to delete group and re-sync in for email address to appear. Or run SPGroupEMail Powershell command that allows you to manually update email property for an AD group in SharePoint.
More info in this post:
http://sandyulrich.blogspot.com/2012/08/update-user-information-email-in.html
