Answer:
Question 1:
Email is an important factor in business communication, and hence, it is a significant part of business. It is convenient to read past messages and save contacts through email.
Question 2:
Subject line is the purpose of email, of why is one writing an email. It is a tagline and is kind of a first impression to a reader.
Question 3:
The most important rule in email writing is that it should be convenient enough to read, and the formatting should be done very clearly.
Hope this helped!
~AH1807