When I worked in an office, I used to be pretty good at timely email responses. I soon learned to write a batch file that held them back, and mailed them all out 30 minutes after quitting time.
It really cut down on people expecting a same-day response all the time, and coming to see me to ask if I got their email. And who knows, maybe some people thought I was actually there at 5:30, working late.
In my view, email was one medium that never really settled down to a common etiquette or set of expectations for its use.