It's a little thing, but it's surprising how many people don't - if you've got a lot of work on and a someone's emailing you about something that isn't a priority at the time it's far too easy not to bother replying immediately. It's much better for your client if you just take 20 seconds to shoot back an email to manage their expectations explaining that you've seen the email and you'll aim to take a look and get them a response in the next x days/hours/etc

Sadly that bar is rarely met. One vendor answers support emails within 24 hours... usually closer to 24 hours. That's a problem when one of our contractors is waiting. If our contractor walks it's $10K to replace him or her.