gender - Referring to someone when you only know their last name

Our company occasionally has to write letters to a third-party in response to a complaint. There are times when we only know the complainant's last name (usually with first initial). Typically, we would write something like "Mr. Jones signed up for service on..." or "Mrs. Smith contacted us on...".

If we do not know the person's gender, what is the appropriate way to refer to the customer? Would we just use "complainant" or should we ensure that we use the customer's name? EX: "The complainant contacted us on..." vs. "J. Smith (the Complainant) contacted us on..."?