The 1099-MISC is issued by you and reflects payments you made in the year 2017, period. It's an expense to you in 2017. If mail delay caused the check to not be received by the contractor until 2018 then he's got a "timing difference" issue he has to address on his own income tax return. If he actually received the check in 2017 but delayed depositing it until 2018 it's income to him. Either way, it's not an issue that concerns you.
Tom Young
