In the late 80s, we actually published a book called !%@: A Directory of Electronic Mail Addressing and Networks, which covered how to address email across 190 distinct networks. (The title !%@ was homage to some of the many special addressing characters that were used before the @ crowded out all the others.)
A big thank you is due to my parents, who gave birth to me in such when and where, that I didn’t have to learn “190 distinct networks” and ways to send email through all of them.