Email

Derek Bridge

Department of Computer Science,
University College Cork

Email

Aims:

• to know how email is sent using SMTP
• to know how email is accessed (a) directly, (b) using POP, (c) using IMAP, and (d) by webmail
• to know how attachments are handled by the MIME standard

The challenge

• How can we send email to users even if they are not connected to the Internet at the time mail is sent?
• Solution:
  • use separate protocols for delivery and for user access and retrieval
  • typically separate the servers (& software) that send and receive email from the end-user hosts (& software) on which email is composed and read

Mailboxes and mail agents

• Every user has a mailbox
  • This is one or more text files, containing all your emails
  • It is stored on your local email server, not your PC/laptop
  • But you might download copies of the emails to your PC/laptop
• Let's use mail agent to refer to the program that you use to compose and read emails

Sending an email

Sending directly from the email server

Sending an email

Sending from the user's PC or laptop over a LAN using, e.g. MS Outlook, Outlook Express, Eudora, Thunderbird, Apple Mail

Sending an email

Webmail (e.g. Squirrelmail, hotmail, gmail): sending from the user's PC or laptop over the Internet using a web browser

Simple Mail Transfer Protocol (SMTP)

• The protocol for both steps is the Simple Mail Transfer Protocol (SMTP)
• Note how the local SMTP server acts first in the role of server, then in the role of client
• SMTP uses TCP for transmission, and SMTP servers listen on well-known port 25

Accessing and reading emails

Reading directly on the email server

Accessing and reading emails

Downloading to the user's PC or laptop over a LAN to, e.g. MS Outlook, Outlook Express, Eudora, Thunderbird, Apple Mail

Accessing and reading emails

Webmail (e.g. Squirrelmail, hotmail, gmail): fetching a web page of emails over the Internet using a web browser

The Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP)

• A user can download email using either the Post Office Protocol (POP or POP3) and the Internet Message Access Protocol (IMAP)
• Both use TCP; both use simple username/password authentication
• POP3:
  • Servers listen on well-known port 110
  • By default, POP3 clients send a delete message after download
• IMAP:
  • Servers listen on well-known port 143
  • By default, IMAP clients do not send a delete message after download
  • An IMAP client can send commands asking the server to organise the mailbox

Email addresses

An email address comprises three parts:

1. The name or numeric IP address of the host where the user's mailbox resides:
   • his/her local SMTP server, not his/her PC or laptop
   • DNS lets us use a different name, e.g. cs.ucc.ie not csmail.ucc.ie. Why?
2. The @ symbol
3. A user identifier:
   • Typically, his/her login id on the local SMTP server
   • But aliases can be set up on the local SMTP server

Email Formats: RFC 822

An email comprises three parts:

1. A header, with fields such as:
   • From
   • To, CC, Bcc
   • Message-ID
   • Subject
2. A blank line
3. The message body

Email Formats: MIME

• The Multipurpose Internet Mail Extensions (MIME) standard:
  • allows emails written in languages using character sets other than ASCII, and
  • allows attachments of various kinds
• MIME specifies types of data and ways to encode them as ASCII

MIME types

Email headers

• If no attachments,
  • Content-type: e.g. text/plain, text/html
• If one or more attachments,
  • Content-type: multipart/mixed, and the boundary delimiter

Email body

• For each part,
  • Content-Id
  • Content-Description
  • Content-Type
  • Content-Transfer-Encoding: 7bit, quoted-printable, base64

Example

From: Ann Sender <ann@someplace.org>
To: Ben Receiver <ben@somewhereelse.com>
Date: Mon, 12 Nov 2007 13:28:19 -0800
Subject: Photo
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="exampledelimtext123****"

This is a multipart message in MIME format

-exampledelimtext123****
Content-Type: text/plain

Ben, here is the photo you wanted me to send you.
Talk to you soon,
Ann.

-exampledelimtext123****

Content-Type: image/jpeg; name="myphoto.jpg"
Content-Transfer-Encoding: base64

SDc9Pjv/2wBDAQoLCw4NDhwQEBw7KCIoOzs7Ozs7Ozs
zv/wAARCADIARoDASIAAhEBAxEB/8QAHAAAAQUBA

-exampledelimtext123****