A way to do this with email and your calendar: go full Yesterbox. Well, full Yesterbox + Boomerang. You limit your email time to one block a day, every day, same time. First, you jam through the previous 24 hours of messages to quickly archive, respond, delete, etc. — whatever you can do quickly. You mark the things that need a longer or more thoughtful response. Later in the day, you have a second block of time for writing those responses. Finally, you use Boomerang to hold your outgoing mail until the next morning, just before your email block.
I’ve done this a few times over the past couple of years. It’s an awesome system, but always seems to break down after a few weeks. It’s hard not to peek at your email, especially on your phone, and feel the false urgency of immediate response.
An email client (or better yet, mailserver) that sends/delivers only once a day would be awesome.