|Mail server computer (analogous to post office boxes) - millimail.org.uk|
|Internet (24 hour connections)|
|Internet service provider's (ISP) computers (gateway)|
|Internet service provider's modems|
|Telephone line (connections made when used)|
|'Jane sends Sarah an email and Sarah replies' - a scenario to explain email connections|
1 Jane connects her computer to the ISP (dials up) this extends the Internet to her computer via the ISP's gateway computer.
2 She runs an email client program, and after typing her ID and password, the program communicates with a mail server program running on the mail server computer.
3 If there is mail waiting for her it is copied (or possibly moved) from the mail server to her computer for her to read.
4 She composes a new message on her computer (possibly in advance). When ready she addresses the message to sarah (email@example.com) and presses the send button.
5 The message is transmitted to the mail server and is stored in Sarah's post box.
6 Sometime later, Sarah connects (via another ISP) and uses her mail client program to check her mailbox and the message is transmitted from her post box on the mail server to her computer.
7 She may disconnect, read the message 'off line' compose a reply and then reconnect the next day perhaps and post the reply for Jane to collect in the future.
Notes: The connections here are through the telephone. In most universities and possibly libraries, schools and other institutions including businesses, the connection is more permanent and the telephone may not be involved. In these cases the service provision will appear direct and seamless to the user - it would be as if the computer were always connected.
Similar mechanisms allow web pages to be accessed from web servers and all the other internet services.