“To the Ephesians” (Eph. 1.1), in preserved biblical texts, is most likely a later addition inserted into Paul’s letter. It can be said with certainty that copies of this letter found its way to Ephesus since Paul meant it as a circular letter (see Col. 4.16). Simon Peter, in his last correspondence to the churches, was aware of Paul’s letters and endorsed them as scripture (2Pet. 3.15). This reference shows that the churches were making copies of these new scriptures and sharing them with other Christians. The scribe, who copied the text of the letter to the church at Laodicea, probably felt the need to designate it, and since his copy was in Ephesus, thought the letter was addressed to this church. Every church in the region, of course, wanted to retain a copy for itself, and the route of transmission, ultimately, became obscured.

Paul ministered in Ephesus for nearly three years and knew the elders of that church intimately, as Acts 20.16ff so poignantly reveals. Eph. 1.15 states that Paul only “heard about their faith,” a statement hardly comporting with Paul’s trials and ministry in that city, along with the Acts 20 episode. There is not one personal reference in the whole of “Ephesians” which diverges from his pattern of personal address in his other epistles. Paul evidently thought his letter to Laodicea contained important information, which slightly diverged, from what he wrote in the letter “Colossians.” By comparing the texts, the Colossians would probably obtain better insight to what Paul was trying to convey: After this letter has been read to you, see that it is also read in the church of the Laodiceans and that you in turn read the letter from Laodicea (Col. 4.16 NIV).

Here is a very nice map and good background concerning Laodicea:

The cities of Hierapolis, Laodicea, and Colosse–all of which are referenced in Colossians 4:13–were located within about 12 miles (19 km) of each other along the Lycus River in the region of Phrygia. Two main Roman roads heading east joined at Laodicea and continued on to Apamea and Iconium. Though Paul almost certainly passed through…

Cities of the Lycus Valley — Bible Mapper Blog