The address of an email will depend purely on the recipient.
For example, if I was emailing the local council regarding a noise complaint, I would need to write in a civilized manner using sensible wording and punctuation. It is strictly forbidden to use abbreviations in these forms of emails as it is unprofessional.
This is an email that I sent to the photography lecturer at Middlesex University, I had to make sure that my terminology and use of punctuation was formal in tone. It is also important to give the email a subject, not only does this give formality to the email, it also gives the recipient an idea of what they are about to read.
It is unlikely that we will use emails to send messages of a more laid-back nature (more likely social media, texting), but this would be an example of an informal message that you would send to a family member or friend. It is brief and bears no subject, as it is a casual email that does not need to be read immediately.