How Random email generation and capturing work

I would like to understand how can I capture emails sent to different random email ids generated by server in one inbox to run analysis on those emails something like this website does : http://ift.tt/10GLh1C

Here , with each page referesh, you would notice a new random email id is generated. If an email is sent to this random email id, the mail-tester server captures that email, assesses it using spamassassin and generates a report. I want to understand how can we capture emails sent to so many different random email ids in a single inbox so that they can be assessed by spamassassin or any other utility.