So says Dave Thomas at JobMob:
It is not uncommon for job ads, especially those online, to come with a P.O. Box or generic email with which to reply. For all you know, it could be your present employer posting an advertisement for help. Make sure you know the source of the help wanted advertisement so that you’re not caught inappropriately sending a resume to your current employer. If you do, chances are you will hear about it.In other words, buyer beware. Sounds like good advice to me.