"To whomever it concerns" vs "To whom it may concern"?

After completing a graduate program I returned to my hometown to be near my family. I am in the process of looking for a job. One position that is right up my alley is posted on craigslist with a blind email. I do not know exactly which business has made the listing and so I do not know for certain who the HR manager is. Is it more correct to begin a cover letter with "To whomever it concerns" or "To whom it may concern?" Or, should I err to "Dear Sir or Madam?" Any input would be appreciated.