[watwut]

I don't think you was horribly inappropriate, tho you really was condescending. You was mostly quite a lot unstrategic and you did not tailored your communication to audience.

The audience is not-too-technical requiter. You introduced term he did not knew (posix) and sent him wall of text explaining history of unix/linux. First, even tech people use terms Linux and Unix as a synonyms for Unix-like daily, so there is not need to go full nerd on someone who is not into it. Local experienced senior could easily have said "we need Unix and Linux experience" and requiter then simply attempts to fulfill that requirement. Local experienced seniors would rather change job then manage communication with dozens of potential interns, you cant expect first contact to be like them.

Second, history of tech wall of text will not work even if you are the most polite and humble person in the world. People don't read long detailed explanation even when they actually need them. They want to move to next mail as soon as possible. If the long text where you are lecturing someone contains just a hint of condescension, even if unintentional, people wont read it and will react negatively (which happened by the end of conversation). It is his bad not yours, but such reaction is still fairly predictable.

Just say that yes, you worked with Linux and Unix systems and leave out historical details. When you are looking for doctor, you don't expect lecture on correct latin names of body parts and their history. You expect them to translate from human speech to doctor speech. It is same thing.