Verse 21. The salutation of me Paul with mine own hand. It is evident that Paul was accustomed to employ an amanuensis in penning his epistles, See Barnes "Ro 16:22"

though he signed his own name, and expressed his Christian salutation in every epistle, 2 Th 3:17; Col 4:18. This gave a sanction to what was written; was a proof that it was his own, and was a valuable token of affectionate regard. It was a proof that there was no fraud or imposition. Why he employed an amanuensis is not known.