1 Corinthians 16:21

Paul makes it clear that he has written the subscript with his own hand.

To avoid any suspicion of forgery, Paul signs the letter with his own hand.

"The salutation of me Paul with mine own hand;" intimates that the Epistle was composed with great seriousness; and therefore he added,

He made a greeting in his own hand, in order to show that he attaches great importance to the epistle.

He adds his greeting, and concerning this he does two things. First, he puts down a title of the greeting, saying, my greeting, namely is written, by my own hand, by Paul. And he did this in his letters on account of some who wrote false letters under the name of the Apostle. Hence, so that they would not be deceived, after the letter was written by someone, the Apostle writes afterwards at the end in his own hand.