Many changes have taken place in the world over the past 20 years. Of those changes, perhaps the most deeply felt was the introduction of the internet.
Let me use a comparison to make things clearer.
When you talk to your mum about your day at school, do you speak to her in the same way that you speak to your friends about what you did over the weekend? Probably you don't. Would you speak to your little sister the same way that you would to your boyfriend or girlfriend? Not likely. This is because different words, tones and styles are required for different situations.
In other words, reading e-mails shouldn't be treated the same as reading a short story assigned by your teacher at school.
Characteristics of e-mail
E-mail is an informal way of communicating so the tone can be relaxed.
They can be long or short - the writer decides.