The word 'etiquette' reminds me of vicarage ladies discussing which way their pinkies ought to point when holding a tea-cup, but I use the word here with respect to the problem of friends who don't, can't or won't, respond.
I don't email much, and typically my 'you have mail' box may have anything between zero and three new emails each morning. I know people who apparently receive as many as 80 a day, excluding advertising! (How such a phenomenon occurs I'm not sure. They must be very talkative.)