ISB ~ 2 Thessalonians 3 v17

I, Paul, write this:Paul takes up the pen to write the final greeting with his own hand, having dictated the rest of the letter to a secretary. His personal signature and remarks authenticate the epistle as a genuine apostolic work (1 Cor 16:21; Col 4:18). He stresses the point here because the Thessalonians may have received a forged letter that alleged to be from Paul but was not (2 Thess 2:2).

every letter of mine:Implies that Paul was in the habit of sending out letters. The statement strikes many scholars as odd, since the Thessalonian letters are the two earliest NT writings that come from Paul. Some take this as evidence that someone other than the apostle—most likely a later disciple, familiar with the corpus of Pauline epistles—must be the real author of this letter. The argument is ingenious, but it must be remembered that not all of Paul’s correspondence necessarily made its way into the collection of NT books. It is possible that some of his letters were neither canonized nor preserved (see, e.g., 1 Cor 5:9 and Col 4:16). The existence of noncanonical letters written during Paul’s early ministry is not established with certainty, but it cannot be ruled out.