It can be reading or writing, but you probably spend a lot of your day hanging out in your email inbox. Sure, they're an essential part of doing business, but they can also take you away from other important tasks and eat into valuable working hours.
Emails can be incredibly distracting. No sooner have you sent a reply and got stuck into another task than another little ping comes in and you're back to the inbox. Studies have shown it can take a full twenty minutes to regain your focus once you've been interrupted, so imagine how much time you actually spend concentrating amid the barrage of notifications.
They can also be incredibly stressful. Whether they're red-flagged or not, many people feel obliged to give an instant reply or risk being seen as inefficient or rude. There are also the anxieties around emailing the wrong person, keeping up with multiple conversations, even worrying you're not being cc-ed into an important thread.
They also mean we miss out on more productive forms of communication. Speaking face-to-face or over the phone are always more effective channels of communication, because you can say what needs to be said and convey emotion. Emails are slow to read, slower to write and cannot give any clues as to a person's tone or intent.
If you find large chunks of your day are given over to your inbox, take a step back. At least take an hour of the day where you mute the pings, get away from the screen and work 'in the real world' with complete focus.
Emails can be incredibly distracting
They can be stressful
You miss out on more effective communication