ChristinaS
Expert Alumni

Deductions & credits

Yes, you can deduct it if its cashed. The check mailed date concept works on the assumption that the check in question is eventually cashed. You won't get a deduction in 2016 if the check needs to be replaced with a new one UNLESS YOU HAVE PROOF that you mailed the check (like Certified Mail).
The IRS does not offer explicit guidance on this issue. This is general practice.