My response:

“For a business email to a potential client whom you don’t know, I would lean toward formality.  I write such emails like traditional business letters, but without the mailing address.  If you want to include the mailing address (yours and his), that would be fine too.”

Call me old fashioned, but I believe that every business relationship calls for a degree of formality.  Mistake a customer for a friend and sooner or later something will go off the track.  This is true, even now in the Age of Twitter.

Prospects, customers, and colleagues deserve a level of respect that should be in play at all times, especially in correspondence.

A good rule of thumb is to write business email as if you’re writing on your company letterhead.  Don’t fall prey to the prevailing assumption that all email is informal by its very nature.

When in doubt, err on the side of formality.  Mind your manners.  When you approach a company about engaging your services or buying your product, be polite – be respectful.  At the salutation, write “Dear Mr. Jones” or “Dear Ms. Smith.”  At the complimentary close, write “Very truly yours,” “Sincerely,” or “Best regards.”  Sign with your full name and title.

If the reply is informal (”John.  Thank you for writing . . . .”), then he or she has set the tone for future emails.  Respond the same way and you both have settled at the right level of formality without risky guesswork on your part.

Does anyone have more to add?  Please comment.