We can think of mail as being the electronic version of the normal postal service—in fact, let's use this comparison to see how it works in more detail. If we look at the following steps, we'll see they are the tasks that are carried out whenever we send a “manual” letter to someone:
Write the letter and sign it.
Address it to the correct person.
Put it in an envelope.
Send it to the post office in the correct area.
Deliver it to the correct recipient.
All these tasks are also performed when sending email, although we aren't always aware of this because the task may be carried out automatically by the system. To find out how it achieves this, let's work through the above steps, using the “manual” letter shown ...