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.
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Your answer leaves my question unanswered. If I sent my check and they have not cashed it (for whatever reason) can I deduct it? If they lost it, isn't that their fault, not mine? If I send them a replacement check can I use the date of the original check or do I lose the donation for last year?
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.