IMAP runs over port 143. Cisco Email Security will still handle any mail traffic sent to it. So, essentially - where is the traffic coming from? Need a little more context to the question.
You will need to have the ESA sit in-front of any end SMTP mail hosts. The ESA will scan/process the mail messages and then deliver to the SMTP mail host.
Check out the Best Practices section to see if this helps you out:
