"Her letter is accompanied by a question that is not easy."
Translation:Suratnya disertai oleh pertanyaan yang tidak mudah.
I got answers from three native speakers. https://hinative.com/questions/18372085
1) Re: "oleh" vs. "dengan", two of them articulated that using "oleh" in this sentence is grammatically INCORRECT. Just in case, I asked whether "oleh" is acceptable but not so good, or not acceptable and incorrect (definitely no). They chose the latter. The last person didn't mention whether correct or not, but I feel she also prefers "dengan" in this context. Therefore, I believe this sentence should be revised.
Here are the reasons. "Oleh" is usually followed by doer(s) (i.e. human beings or animals). "Pertanyaan" is obviously not the case you can use "oleh". The next question would be why "dengan" among many other prepositions. "Dengan" is used when you describe HOW the action was done.
Another function of "oleh" is a conjunction rather than a preposition (by). "Oleh" in a passive sentence can be interpreted as the same meaning of "karena" (because of), so that "oleh" illustrates the cause-effect relationship. For example, "the flood was caused by the heavy rain" = "Banjir disebabkan oleh curah hujan yang tinggi". In this cause-effect case, "oleh" can be also followed by inanimate objects.
2) Re: passive (di- verb) vs. active voice (me- verb), both of the following sentences are correct and natural. However, the passive sentence is preferred.
"Suratnya disertai dengan pertanyaan yang tidak mudah."
"Pertanyaan yang tidak mudah menyertai suratnya."
Hope this helps!