NIV: Paul, an apostle of Christ Jesus by the will of God, To God’s holy people in Ephesus, the faithful in Christ Jesus;.

Explanation

This is a difficult issue. All translations include “in Ephesus,” even though the three earliest  and best manuscripts do not include the phrase (P46 Sinaiticus Vaticanus). Part of the issue is that parts of the epistle sound as if Paul did not know the Ephesians, and yet he spent over three years there. If it is not original, then the letter was probably encyclical, intending to be read throughout Asia Minor. If the words are original, then Ephesus probably is listed as the main city in the area.

UBS gives the inclusion of the words a C rating.