dear sirs

It is the year 2016. You cannot assume in your business correspondence that any and all respondents are male.

If you have a vending or contracting relationship with a business, and your business contact is a female person, and you would like to continue to have a business relationship in which she arranges to pay you on a regular basis, it would behoove you not to address that person as "Dear Sirs" on your submittals.