When you've written that Very Important Email in your email client and hit "Send", a few things happen that you don't notice:
- Your email client contacts your sending email server and hands off the email to it.
- Your sending email server inspects the email to find the recipient email address.
- The domain from the recipients email address is used to look up the recipients email server in DNS.
- Your sending email server then contacts the recipients email server and hands off the email to it.
- And hopefully at some point, the recipient opens her email client, which contacts her email server, which in turn hands off the email to it.