Adam Clarke on 2 Thessalonians 3:17

The salutation of Paul with mine own hand - It is very likely that Paul employed an amanuensis generally, either to write what he dictated, or to make a fair copy of what he wrote. In either case the apostle always subscribed it, and wrote the salutation and benediction with his own hand; and this was what authenticated all his epistles. A measure of this kind would be very necessary if forged epistles were carried about in those times. See the note on 1Cor 16:21, and see Col 4:18 (note).