Email still being sent to people no longer members

It appears that when we update a task that used to be assigned to someone who is no longer a member on the project (their email/user is not listed as a member or anywhere in Tracker that I can find - we deleted them/removed the user), they are still getting emailed when we update that story. This is luckily not too major a security issue for us, but still, we don't want our company data being sent out to people who are no longer with us.