Could be different things including that it has nothing to do with osTicket at all.
You need to provide detailed information such as how do you have osTicket setup to get email? Piping, cron...???
Hi Mike, I clicked Admin Panel -> Email Settings and Options, I put support@xxx.com as Default System Email. For Incoming Emails, I tick Enable POP/IMAP polling and Poll on auto-cron. For Outgoing Emails, I choose None: Use PHP mail function as Default Outgoing Email. Do you still need any detail information for it? Many thanks.