smallfish So the question is how can I setup osTicket make system identify [username@domainA.com](mailto:username@domainA.com), [username@domainB.com](mailto:username@domainB.com) and [username@domainC.com](mailto:username@domainC.com) are the same user?

Currently you can't. Multiple emails are a unique value in osTicket currently.

smallfish Another question: how can I disable outside the organization users send email to create ticket? Or how can I create a white list just enable [username@domainA.com](mailto:username@domainA.com), [username@domainB.com](mailto:username@domainB.com) and [username@domainC.com](mailto:username@domainC.com) email can create ticket?

You would handle this at the mailserver level.

If you can't do that, then you would create a Ticket Filter.
Go to Admin panel -> Manage -> Filters
Click Add New Filter
On the Filter Rules tab
Select User / Email Address
Select Does Not Match Regex
enter the email addresses I think like:
(email@address1.com|email@address2.com|email@address3.com)
note: you might have to play with the Regex, I'm mediocre at regex.
Click on the Filter Actions tab
Select if you want to reply, or just drop the ticket.