Lightweight Task List mail notification is not received when using Exchange 2007 SMTP server
You have the lightweight Task List configured in Confluence page however when you assigned a task to a user, no email notification is received and it got stucked in the Mail Queue (checked from http://<confluence-url>/admin/mail/viewmailqueue.action page). All other mail notification is working fine.
Exchange server does not have relay enabled for Confluence server.
Enable relay for Confluence server in Exchange server.