The MTA Service is responsible for the routing of email and and out of MailEnable. When an email is accepted by the SMTP or POP retrieval service, the MTA service determines which connector is used to handle the email. For instance, if the email is destined for a mailbox, the MTA service will send the email to the Postoffice Connector in order for it to deliver the mail to the correct mailbox. If the email is destined for a list, the the MTA service will send the email to the List service.
Click here for details about how to configure the MTA service.