Not everyone knows what (nt) means. I have never seen it before and I would consider myself pretty web savvy. I mean it would be pretty obvious once I opened my message, that the subject was the information I needed.
it's like using POD, that has a very specific usage, but is not by any means universally used throughout the web.
Remember, just because you (general) see or use an acronym or abbreviation all of the time, doesn't mean that everyone else does.
I agree. That's why at the place I worked, we made a point to tell new colleagues about the convention. (Nobody told me, but I figured it out eventually.)
I don't think there is any "proper netiquette" that everybody knows and understands.
And if the OP's landlady wanted to remind people--why didn't say something more sensible in the subject line, like "payment reminder"? Insert eyeroll here.
But, now the OP knows what the landlady's "secret code" is.