Colossians 4:16

τὴν ἐκ Λαοδικίας : clearly a letter sent by Paul to Laodicea, which the Colossians are instructed to procure and read. It may be a lost letter, or it may be our so-called Epistle to the Ephesians, to which Marcion refers as the Epistle to the Laodiceans, and which was probably a circular letter. Weiss argues that it cannot be the Epistle to the Ephesians, for that was sent at the same time as this, and therefore Paul could not have sent salutations to Laodicea in this letter. But this is really natural, if Ephesians was a circular letter (and the absence of salutations is difficult to explain otherwise), and if this letter was to be passed on to Laodicea.