The salutation of Paul with mine own hand, which is the token in every epistle: so I write.
The letter was written by an amanuensis (perhaps Silas or Timothy), and only the closing salutation, 2 Thessalonians 3:17-18, by Paul's "own hand" (cf. Romans 16:22; 1 Corinthians 16:21; Colossians 4:18). Wherever Paul does not subjoin this autograph salutation, we may presume either he wrote the whole letter himself (Galatians 6:11), or deemed formal attestation needless.
Which - which autograph salutation.
The token - to distinguish genuine letters from spurious ones in my name (2 Thessalonians 2:2).
In every letter. Some think he signed his name to every letter with his own hand; but as there is no trace of this in any manuscripts of all the letters, it is more likely that he alludes to his writing with his own hand in closing every letter, even in those letters (Rom., 2 Cor., Eph., Phil., 1 Thess.) wherein he does not specify his having done so.
So I write - so I sign: this is a specimen of my handwriting, by which to distinguish my genuine letters from forgeries.