I had a little bit of a hard time setting up the cron job to retrieve mail on my new osTicket.
Here is what I finally figured out.
sudo crontab -e
If this is the first time you have run this it will ask you to select an editor. I use nano.
This will place you in a text editor.
Add this line after the last line (changing the path to match your setup)
*/10 * * * * /usr/bin/php /var/www/html/mywebsite/api/cron.php
This will look for mail every 10 minutes.