How to politely say "I don't know you"?

I received an email from someone I haven't known. It's a letter related to my job, in other words - a business one.
How would you write him an answer highlighting that you don't know this person and your possible actions depend on who he is.
In a hurry I sent something like: "I'm sorry, but I don't know you, so...", but I think it sounds rude.

Solution 1:

You could say:

My apologies, but I don't believe we've met. Before we proceed further, could you please verify that I'm the person you meant to contact?

Solution 2:

"Considering that we are not acquainted..."

"Given the sensitive/confidential nature of this subject..."

"I am afraid this is a company/internal matter."

If you want to keep the same basic form, it's politer to say "I don't believe I know you..." However this leaves a slight bit of doubt, so that he could conceivably come back and say "oh yes, we met, remember?" If you really want to be clear that you don't know the person, use something like the first three above.