Paul's letter to the Ephesians may have been much more of a general letter, for wider circulation. We have a couple reasons to think this. One is that there is nothing written specifically for that group. No mention of names except Tychicus, the would-be letter carrier. No personal greetings at the end of the letter. No specific problems they needed to deal with like in many other epistles.
The other reason comes from an interesting textual variant. Some manuscripts contain the words that we translate "in (or at) Ephesus." Other manuscripts leave it out. The Marcion Cannon (a list of all the books he believed were part of the New Testament) called the letter "To the Laodiceans."
But this sticky problem is not as simple as other textual variants among manuscripts. In this case, early and late manuscripts include "in Ephesus" and early and late manuscripts just drop it. Marcion's Cannon is somewhat early, although borderline in the middle of the period in question. The construction of the sentence is awkward for a greeting if it's just dropped. What's going on here?
F.F. Bruce, an outstanding biblical scholar, sheds some light on the matter. He argues that this letter was probably intended to be widely circulated. The location would be substituted in the spot were it says "in Ephesus." Bruce cites Zuntz who said this was a rather common practice by government and royal letters during this time period. (Bruce, 1984, 249-250.)
Think of it like a mail merge form letter that we might use today. There might be a blank or something like [FirstName] where the substitution is intended to be made. The only evidence that might counter this thinking is Ephesians 5:21-22 regarding Tychicus coming; but that is very light compare to other Pauline letters.
So in Paul's original was there just a blank in the place were we read "at Ephesus"? Not likely. In the royal letters of this period, we can see that the original would contain a city name or region. The person copying the letter going to a different city would make the change.
In the case of Paul'l letter it seems this wasn't widely understood. Most of his other letters were written to specific places. People were copying those letters and circulating them widely (and maybe Paul didn't understand the nature of this circulation). So when Paul wrote a letter intended for wider circulation, people just did what they did with every other letter. They copied it word-for-word and didn't worry about it. In the case where someone might have just left it blank, there may have been an effort to realize that the letter was for wider circulation.
What does this mean for us?
Well, it might be what Christians love about Ephesians. It was written for a wide circulation so it feels so applicable to every Christian. Because it doesn't deal with specific issues of one church, it is easier to apply the writing directly to every church. We don't have to do much to extract the timeless meaning before making application. We could write, "To the saints who are at This Church or That Church or First Baptist Church of Somewhere, and are faithful in Christ Jesus" and the letter would still make perfect sense (with exception to slaves/bond-servants, but that too will make more sense when we get there).
As you read this letter, see it as addressed to your local church and you, as a part of your local church.