Judith Martin — American Author born on September 13, 1938,

Judith Martin, better known by the pen name Miss Manners, is an American journalist, author, and etiquette authority. Martin's uncle was economist and labor historian Selig Perlman... (wikipedia)

I make a distinction between manners and etiquette - manners as the principles, which are eternal and universal, etiquette as the particular rules which are arbitrary and different in different times, different situations, different cultures.
Etiquette does not render you defenseless. If it did, even I wouldn't subscribe to it. But rudeness in retaliation for rudeness just doubles the amount of rudeness in the world.
For email, the old postcard rule applies. Nobody else is supposed to read your postcards, but you'd be a fool if you wrote anything private on one.
The greater the controversy, the more you need manners.
One of the big no-nos in cyberspace is that you do not go into a social activity, a chat group or something like that, and start advertising or selling things. This etiquette rule is an attempt to separate one's social life, which should be pure enjoyment and relaxation, from the pressures of work.