Because of weak eyesight, or the difficulty of his handwriting, or some other bodily infirmity, he did not write his epistles with his own hand, but dictated them to scribes.
For this reason, malicious opponents of the Apostle could easily issue their own writings under his name and thereby harm the truth of apostolic, divinely inspired teaching.
This is what happened in Thessalonica. A forged epistle appeared there, supposedly sent by Paul. It shook the minds of the Thessalonians and produced a strong, though false, alarm in their hearts.
To remove such harmful falsehood in the future, the Apostle agreed to act in this way. At the end of each epistle, he would always add the concluding greeting to churches and persons in his own hand.
Read more (56 words)
That is, he would write the words, “The grace of our Lord Jesus Christ,” and so forth, with his own hand. This is what he always did afterward.
Only the Epistle to the Galatians did he write entirely with his own hand, which he presents as a sign of his special zeal for them (Gal 6:11).