Whenever an email is unable to reach the recipient, you usually receive an email notification called NDR (non-deliverable receipt). In order to make EmailUnlimited recognize those NDRs, it has a list of subject lines that indicate that an incoming email is an NDR. You can edit that list by going to Tools and selecting 'Bad mail (NDR) recognition...'. The following dialog will appear:

EmailUnlimited™ recognizes these returned emails and adds them to a do-not-email list or removes the addresses from your mailing list and the underlying database, depending on your campaign configuration.