What was the last letter written by the apostle Paul?

Thirteen 'epistles' or letters have been attributed to Paul. Hebrews was also at one stage attributed to him, but that is clearly not the case. It was never attributed to him until late in the second century.

At least seven Pauline epistles are widely considered to be genuine, and of these five have been shown by computer analysis to contain exactly the same writing or dictation style: Romans, 1 Corinthians, 2 Corinthians, Philemon and Galatians.

The two that may, with some confidence, have been written by Paul, but do not match the style of Galatians are Philippians and 1 Thessalonians.