You didn't have to run a BBS to do this. Using an offline mail reader like Blue Wave  and downloading QWK files  allowed anyone to do it—and helped keep the lines, as downloading was (relatively) quick compared to reading 'online'.
I initially got "on" Usenet by downloading SOUP files from a local free-net .
In fact, when I finally got email, I wrote a QB program to convert my inbox to QWK format. I would then use OLX to read/reply to emails. I don't recall, but I probably used OLX's Save As to store my replies to text files which I then uploaded to my online mail server.
I used OLX for emails until at least 1999.
Seems you can still get it: