"The Letter of Paul to the Ephesians is a letter, not by Paul, and not to the Ephesians"
Is it a letter? Unlike all Paul's other letters, it contains no personal references or advice specific to a particular church. It has, however, been suggsted that it is a circular letter, intended to be sent to several churches, in turn.
Is it by Paul? Paul is cited three times as the author of the letter, but it is not in the style of the other letters -- 44 words are included that are not used by Paul elsewhere, and the writer has a non-Pauline tendancy to write very long sentences -- for example the whole section from chapter 1 verse 15 to the end of the chapter is one sentence.
Is it to the Ephesians? Early manuscripts do not have a TO label at all, and the early (heretical) church father Marcion thought it was written to Laodicaea.
The document shows many similarities to the Epistle to the Colossians, and appears to attack Gnostic or proto-Gnostic tendancies. The writer seems not averse to using Gnostic terminology himself; there are references to archons, powers, and principalities, and well as several references to multiple heavens. As with Colossians, the emphasis is on the supremacy of Christ above all such concepts.