Our event confirmation emails are not being sent. The PayPal payment is happening, but the Online Registration Confirmation is not happening. What have I forgotten to enable??? We are using CiviCRM ver 4.7.1. I don't know how to find the version of Joomla. Emails DO go out, but only to those who do not owe any money - who have not gone to PayPal. Those who have paid remain in the "Pending (incomplete transaction)" status. We are NOT using "Pay Later".
I've recently had to address this issue.
1) Make sure that the PayPal's "callback" setting is enabled and set to point to your website.
2) Speak with your sysadmin and make sure that calls from PayPal are not being blocked.
3) Look in your CiviCRM logs around the time of the payment to see if there are any errors.
4) Ask your friendly neighbourhood software developer (or me or a CiviCRM partner agency if you don't have one) to add extra logging and debug output to determine if the calls are being received and traced through the system.
5) Check the spam folders - there's a lot of things that you can do if emails end up being sent to spam.
I've experienced a similar issue when for some reason, my hosting environment reverts to an older PHP version (5.3.29). I use SparkPost and it needs 5.4.x so it stops working until I change PHP version. Not sure if it applies to your situation but thought I'd mention.