Tasks List not sending email to AD Groups

Question

Hello,
I have several Tasks list on my farm and all behave the same way.
If I assign a task to an individual person, he will receive email notification.
However, if I assign it to an Active Directory group, no email notification is sent. The AD group does have an email address associated. Using outlook it can receive email.
The email notifications for the task list is enabled. If I check the SharePoint logs, I don't see that SharePoint is trying to send mail to the group's email. If I assign the task to a single person, he receives the mail and I can see in the logs that the
email has been sent.
I have no idea where to continue the troubleshooting.

Answer 1

Try the following:
Verify that the group is an AD security group (not a distribution list) and double check that the AD group is email-enabled.
Check that the group had been explicitly added to a SharePoint group (such as the default Viewers group) with at least read permission to the site and to the list on which the alert was created.
Verify that the SharePoint group’s membership is set to “Visible to everyone.” (People and Groups>SharePointGroupInQuestion > Settings>Group Settings)
Would also be worth checking on the Exchange server to see if there any 'Message Delivery Restrictions' for the AD group:
Exchange Management Console -> Recipient Configuration
Right-click on the problem group and choose properties.
On the Mail Flow Settings tab, double click on Message Delivery Restrictions.
Uncheck the check box “Require that all senders are authenticated”