“17 I, Paul, am writing this greeting with my own hand, which is an authenticating mark in every letter; this is how I write.”
Paul was revealing to the church, who he was writing to, how they can recognize his letters. The reason why is because there were many false apostles in the time of Paul. See below.
[2 Corinthians 11:13-15] 13 For such people are false apostles, deceitful workers, disguising themselves as apostles of Christ. 14 And no wonder! For Satan disguises himself as an angel of light. 15 So it is no great surprise if his servants also disguise themselves as servants of righteousness. Their end will be according to their works.
Also, they (the false apostles) were writing letters to the church in Paul’s name, deceiving the church. See below.
[2 Thessalonians 2:1-2] 1 Now concerning the coming of our Lord Jesus Christ and our being gathered to him: We ask you, brothers and sisters, 2 not to be easily upset or troubled, either by a prophecy or by a message or by a letter supposedly from us, alleging that the day of the Lord has come.
Paul wanted to make sure that the Thessalonians would not be deceived by believing a false gospel.