Why aren't send update request emails coming through to my Outlook account?

I am a Project Manager (super user) for a company and have enabled the send update request functionality in Clarizen.  I have attempted to send update request emails for tasks to resources and they are not receiving the email.  I did a test task and assigned a test user account (my personal Yahoo account) as the resource for the task and I received the email.  What setting do we need to change on our Outlook Exchange server to allow these emails to come through?  All other emails sent from Clarizen come without a problem.