I have a few questions in mind to ask along these lines but am waiting to see what happens with this question. (I voted to keep open.) This doesn’t seem to clearly fall into any of the categories listed here (except possibly “historical context”?) , but we do have a tag called “authorial intent” which would fit nicely with these sorts of questions. It seems to me that they should be on topic (although I’m biased – I want to ask them!), but I’d like clarify.
We've generally allowed these kind of introductory matters such as provenance, authorship, dating, or even canonicity. I don't see why audience would be would be any different from those. The intended audience is a key part of the interpretive process for certain hermeneutics. Our FAQ does seem to exclude a number of items which are, in fact, considered on-topic here. For instance, another glaring omission from that list is textual-criticism, which is one of our most popular tags.
In this particular question's case, it would maybe have been better to ask about a particular letter. Expanding it to thirteen letters makes any single answer difficult to defend for having to cover so much turf. But on the whole, I don't think it is off topic.