CiviMail not sending emails - Joomla 3.6 or 3.7 & Civi 4.7.18
It's not easy to get it working properly indeed.

Sending a "normal" email and civimails follow a different path. The former is simple: you call the "send an email" you configured (usually the mail() function, but can be configured, eg to use smtp directly), wait until it's done and that's it.

This doesn't scale and if you aim, like civimail does, at sending lots of emails, you need to queue these emails, possibly split them in separate batches and have a cronjob that process them in the background to send these emails

And were it becomes even more complicated is that you have a lot of different ways to run that cronjob, because civicrm aims at serving a lot of different configurations.... and each have lots of opportunities to have something not working.

As you are using joomla, the "common solution" we use (drush) isn't an option. You do not detail your system, do you:


have access to the command line (ssh)
can set up cronjobs
have php-cli installed and with the same configuration as the php used for your server?


You haven't described enough how have configured CiviCRM. If this is not how you are using it, could edit your question and add these details and what you get if you run

path/to/php /path/to/civcrm/module/bin/cli.php -s site -u user -p password -e Job -a execute


As described: https://wiki.civicrm.org/confluence/display/CRMDOC/Managing+Scheduled+Jobs