The default subject line that goes through when someone sends an email to a Job Poster is like this:
Application for job "Title of the job"
With "Title of the job" being based on whatever the actual title of the job is in JobRoller. So, if you're placing all the ad listings yourself, you could of course choose to include the business name in the Title itself.. This would be the easiest way, otherwise you would be looking at some specific customization to the code which could allow JobRoller to be able to pick up which business created the listing in the first place in order for it to appear in the subject line.. I imagine working that out could be quite complex, but if you have the coding skills to work that out then you can certainly make the changes if you wanted to as the theme is not encrypted, making it free to be customized.