I heard from a lawyer friend who works in a high profile law firm. He told me a partner emailed an associate a question late in the evening (like 9 or 10pm, maybe even on Fri night) and didn't hear back a reply that same night.

The partner blasted an email to all in the firm publicly flogging the associate lawyer the next day...

Considering lawyers in a law firm are not in a union, I guess such 'ban' on email won't be happening anytime soon...