Yes. According to IRS guidance:
Checks. A check you mail to a charity is considered delivered on the date you mail it. Before deducting, however, I would verify with the charity that the check was received. You are not in the wrong for a deduction, but you also should ensure that the money is literally paid to the organization. If they lost the check, you haven't paid anything.
Page 13
https://www.irs.gov/pub/irs-pdf/p526.pdf
