Send Alerts to External Users, AD Accounts within AD Security Groups

We create external users in AD, then apply them to a security group.  We then apply the security group to SP. Everything works fine except for alerts. The same practice works fine for internal users.  On the external users, SP picks up the email address, allows the alert to be created, but doesn’t send the email. We found that if we add the External User's AD account directly to SP in lieu of the security group they then receive the emails.

We would really like to avoid adding the users directly to the SP Sites, the AD Groups is much more convenient. Are we missing something, or is this simply something that we have to work around?

SharePoint Foundation 2010.