unusual situation for starting a letter

scenario: writing a letter to someone, the name of the recipient is not 100% known - a name is known but (part of) the reason for sending the letter is to ask if it is being read by the person intended

addressing it to this name doesn't seem right, incase it's not them

but nor does dear sir, because 1) if it is indeed them, it would not be correct and 2) it is not formal enough for this, but formal enough to not use "hello"

however, not first letter - contacted (the correct person of course) before, but not contacted the wrong person (if wrong) before

what would you do?

thanks