Albert Barnes Commentary 2 Thessalonians 3:17

"The salutation of me Paul with mine own hand, which is the token in every epistle: so I write." — 2 Thessalonians 3:17 (ASV)

The salutation of Paul with my own hand. (See Barnes on 1 Corinthians 16:21).

Which is the token in every epistle. Greek, sign. This signature is a sign or proof of the genuineness of the epistle. (See Barnes on Galatians 6:11).

So I write. This refers to some mark or method Paul had of signing his name, which was well known and would easily be recognized by them.