1. Your email account: When you have an email account, you have a user name (like Jacky555) at some domain (like Earthlink.net or AOL.com), some space for your incoming messages on an email server, and the ability to send outgoing messages through (maybe a different) email server.
2. Your email client software: Just having an account isn't enough.  You have to install (or maybe upgrade) a browser/email software like Netscape or Opera, or a standalone email client like Eudora or Outlook Express.  Then you must make the correct settings so this software knows where it can talk to the mail server.  There are 2 main settings, one for your outbound messages (SMTP) and one for your incoming messages (POP).  The settings are server names look like smtp.earthlink.net and mail.earthlink.net.
3. A connection to the internet: Dial-up, cable, satellite, or DSL internet access is needed to transmit the message from your email client to the outbound server, so it can eventually get to the intended recipient.

1. My email client adds the "email headers" to the top of her message, and the message text and any attachments are transmitted to the JackySuch.com SMTP server through my internet connection.  A good explanation of how THAT happens, a TCP/IP transmission across the infrastructure of the internet, is explained here* at HowStuffWorks.com.
2. The server confirms that I have the right to send mail.  This may be with a login and password that the email client automatically sends, or by typing the password into a password dialog box.
3. The server accepts the message for sending and requests a "look-up" to make sure the domain name exists, and to learn its IP address.  This is because each internet server has an Internet Protocol (IP) number address, like 64.81.150.215, and the message travels by number address, not name.  The "look-up" is asked of the Domain Name System servers (DNS), which keep a table of the server names and numbers.  All the DNS servers do is answer other servers with this information. There are 14 "root" DNS servers controlled by the domain name registries, and thousands of smaller DNS servers distributed all over the internet to answer these questions.
4. The message gets sent to the server at that IP number address.

1. The POP server reads the headers and checks to be sure that Michelle555@nothotmail.com is a real address 
2. It might compare the sending address to a list of know spammers (and delete the message.)
3. Then it checks that the inbox is not full (over the limit on hard disk space) to accept messages.
4. The message is then "dropped" into Michelle's mailbox, waiting for her to retreive it.

1. When Michelle hits "Get Messages" on her email program, it sends a request to the POP server at nothotmail.com. 
2. She will have to "authenticate" herself to the server by telling it her username and password, either automatically through the software, or maybe by typing it in.
3. The server will transmit the message through her internet connection to her computer's email client inbox where she can read it.

Definitions included on this page: authenticate, DNS, email account, email client, email server, IP, POP, SMTP.