Your colleagues are busy. In the first split-second after they open your email, they will scan the message to determine whether or not they have time to read it that moment.
Imagine your recipient taking a still photo of your email and forming their first impression of it. What do you want in that photo?
If your email’s text is too long to be viewed completely within the initial screen, your recipient will be instantly turned off. If they’re focused on anything else at the moment, chances are good they will close your message to deal with it later. Worse, when they re-open the message, it will turn them off all over again.
Your message should be only as long as you need to communicate the information to your recipient. If you need more than an entire email screen, you should instead communicate by phone or face-to-face.
A few more tips for writing better emails at work. Hope you find them useful.