Letter to an important business leader

I met an important business leader from Japan yesterday and wish to write him a letter. Should I address it to his last name followed by -san, or the more formal -sama? Or would it be proper to address it to his first name?

Are you writing the letter in English or Japanese? In any case, unless you got to talk in first-name basis upon your encounter, family name + sama would be the proper way.

Thank you for your help!

Paul, if you are writing the letter in English, write it in exactly the same way you would when writing to a native English person. Using "san" or "sama" with names when writing or speaking in English sounds affected and can even come across as condescending.