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.