How does email work?

Lisa opens her email program (Outlook Express), uses her email account, lisa@otherisp.com, and writes an email message to bob@comentum.com.

Her Outlook Express sends the message through TCP port 25 of Lisa's Internet connection, then to the other ISP's SMTP server (still on TCP port 25).

The other ISP's SMTP server finds the destination's SMTP server (in this case, Comentum) and sends the message out to Comentum's SMTP server (through TCP port 25 of her Internet connection).

Comentum's SMTP server sends the message to Comentum's POP3 server where the message waits for Bob to pick it up.

Bob opens his email program (Outlook Express) and when he clicks Send/Recv button, his Outlook Express sends a request through Bob's Internet connection to Comentum's POP3 server (through TCP port 110) and downloads Bob's message to Bob's computer where he sees Lisa's message.

• IMAP

For example, the client will download all the messages and store their complete contents on the local machine (just like it would if it were talking to a POP3 server). The messages still exist on the IMAP server, but you now have copies on your machine. This allows you to read and reply to e-mail even if you have no connection to the Internet. The next time you establish a connection, you download all the new messages you received while disconnected and send all the mail that you wrote while disconnected.


References:
http://www.comentum.com/how-email-works.html
http://computer.howstuffworks.com/e-mail-messaging/email5.htm