Setup an email account.
Set up osTicket to check that email address.
Set up Cron or Windows Task Scheduler to run cron.php every few minutes.
Have client email the address that you setup.
more information: http://www.osticket.com/wiki/Email_Settings(http://www.osticket.com/wiki/Email_Settings)
Even more information: http://www.osticket.com/wiki/POP3/IMAP_Settings(http://www.osticket.com/wiki/POP3/IMAP_Settings)