Things I would check.
Look under Store Manager > Order History, view the order detail and the PayPal log will show if it received any IPN notifications. If you see IPN notifications in the log then it probably really is just an email problem but if not then you probably don't have IPN configured correctly.
For email problems, I would first make sure you get it working in other features like the contact form. See Email Configuration.
Also make sure you have configured the from address in Store Settings. You might laos look in the mojoPortal l under Administration > System Log to see if there are any email related errors being logged.
Hope that helps,