Where is que'd mail stored at?

Assuming that there were no problems (errors,filters, forwarders etc) and the mail has been successfully delivered to the mailbox, it should be available at /home/username/mail/domain.com/test/

Of course, the variables in the above path have to be substituted accordingly.
'Username' should be replaced by the actual cPanel username.
'domain.com' should be replaced by the actual domain name and 'test' should be replaced by the first part (preceding @ symbol) of the email address in question .

 

Also, be careful calling it "Queued" mail, as there is actually a two mail queues, so mail being delivered is no longer "queued". That could add to some confusing if you use that word talking about delivered mail.