E-mail, SMTP,
and MIME types
Lesson 26 - (1 of 41)
Outline:
E-mail, SMTP, and MIME types
• Mail Systems:
– Terminology
– Delayed Delivery
• Names and Aliases:
• Controlling E-Mail
Messages:
• Internetworking and Mail:
• Mail Gateways:
• Mail Addresses:
• Simple Mail Transfer
Protocol (SMTP):
• Post Office Protocol (POP)
• E-Mail Application
Program Interfaces:
– MAPI
– AOCE
• MIME Extensions
• MIME Types and Subtypes
• S/MIME Content Type
• MIME Multipart Messages
Lesson 26 - (2 of 41)
Mail Systems:
• Interactive Client / Server Message Exchange:
– Messages can be generated and delivered using
an electronic mail client.
– Those messages are then passed point-to-point,
from e-mail server to e-mail server, until
delivered or rejected.
Lesson 26 - (3 of 41)
Mail Systems:
• Mail systems differ from normal network delivery
since the absolute destination can be down or the
mail recipient may not be on-line (delayed
delivery).
• Similar to conventional mailing schemes, e-mail
allows a server to place successfully delivered
mail in the recipients mailbox.
Lesson 26 - (4 of 41)
Mail Systems:
Terminology
• Terminology:
– E-mail (Electronic Mail) - most computers
support e-mail of different types and over
different operating platforms (delivery times
range from seconds...hours or days.)
– Snail Mail (Conventional Mail) - post office is
used and manually delivered (takes several
days).
Lesson 26 - (5 of 41)
Mail Systems:
Delayed Delivery
• Spooling or Mail Queues store mail messages the
user sends in a private area, which include the
following information:
– ID of sender
– ID recipient
– Destination machine address
– Time of deposit
Lesson 26 - (6 of 41)
Mail Systems:
Delayed Delivery
• After spooling messages, the system then initiates
the transfer to the remote machine as a
background activity.
• Spooled messages can be sent at various times in
block amounts (transfer process sweeps through
the spool area periodically).
Lesson 26 - (7 of 41)
Components of an E-Mail System:
user send mail ......
user
interface
user reads mail
......
outgoing
mail spool
area
mailboxes
for
incoming
mail
client
TCP connection
(background
transfer) for outgoing mail
server
(to accept
mail)
TCP connection
for outgoing mail
Lesson 26 - (8 of 41)
Names and Aliases:
• Recipients specified by pair of identifiers:
– Mail destination machine name.
– Mailbox address on that machine.
• Mail machine names and mailboxes can be
assigned independently of other names assigned to
a machine (position of employment or
responsibility).
Lesson 26 - (9 of 41)
Names and Aliases:
• The domain name system (DNS) supports a
database and query type for mail destinations.
• This makes it possible to de-couple mail
destination names from usual domain names for
machines.
• Some e-mail packages require explicit DNS
commands be used when sending e-mail
(VAX/VMS requires MX%”name”)
Lesson 26 - (10 of 41)
Controlling E-Mail Messages:
• Methods of sending e-mail:
– One-to-one.
– One to a group of e-mail addresses (all
specified each time).
– Mail Aliases
– Mail Exploder (listservers)
– Majordomo (automated response e-mail)
Lesson 26 - (11 of 41)
Controlling E-Mail Messages:
• Mail Alias Expansion (mail forwarder):
– Allows the local site to map identifiers used in
mail addresses to a set of new mail addresses
(electronic mailing lists, mail forwarding).
– Files can be setup in UNIX accounts to allow email messages to automatically be forwarded to
another specified e-mail account.
Lesson 26 - (12 of 41)
Controlling E-Mail Messages:
• Mail Exploder:
– Accepts one incoming message and sends it to a
large set of recipients.
– Acts as an alias for a group destinations where
a copy is sent to each member (listservers).
Lesson 26 - (13 of 41)
alias
database
user sends mail ......
user
interface
user reads mail
......
alias
expansion
and
forwarding
outgoing
mail spool
area
mailboxes
for
incoming
mail
server
(to accept
mail)
client
(background
transfer)
Lesson 26 - (14 of 41)
Internetworking and Mail:
• Commercial e-mail systems differ from TCP/IP in
two crucial ways:
– TCP/IP on the Internet creates a universal
delivery service.
– E-mail systems built on TCP/IP are more
reliable (TCP provides end-to-end
connectivity).
Lesson 26 - (15 of 41)
Internetworking and Mail:
• Mail systems that use end-to-end delivery can
guarantee that each mail message remains in the
sender’s machine until it has been successfully
copied to the recipient’s machine.
Lesson 26 - (16 of 41)
Mail Gateways:
• Mail Gateways (mail bridges):
– Are not end-to-end, but send mail across
intermediate machines that forward the mail
messages.
– Advantage - provides interoperability between
various e-mail systems and networks.
– Disadvantage - introduces unreliability.
Lesson 26 - (17 of 41)
Mail Gateways:
• TCP/IP mail standard has two main parts:
– Specifies the format for mail messages and
their headers.
– Specifies the details of electronic mail
exchange.
Lesson 26 - (18 of 41)
Mail Addresses:
• Connected Internet addresses are simple:
(localpart@domain-name)
– Localpart - usually a user’s ID or account
name.
– Domain-name - officially registered with the
IAB, this name is usually structured to reflect:
• Machine name.
• Site name.
• Top-level domain.
Lesson 26 - (19 of 41)
Mail Addresses:
• Outside connected Internet addresses are complex
since mail address mapping functions are local to
each machine.
• They are not always in domain-name format.
– dzt!mttec@mt.att.com
– comer%purdue.edu@relay.cs.net
Lesson 26 - (20 of 41)
Simple Mail
Transfer Protocol (SMTP):
• Simple Mail Transfer Protocol (SMTP):
– This protocol focuses specifically on how the
underlying mail delivery system passes
messages from machine to machine.
– It is an official TCP/IP standard application
protocol.
Lesson 26 - (21 of 41)
Simple Mail
Transfer Protocol (SMTP):
• It provides readable text transcripts of clientserver interactions in the mail header and integer
values for commands:
– MAIL FROM
– RCPT TO
– HELO
– 250 OK
– QUIT
Lesson 26 - (22 of 41)
Example SMTP Interaction:
• S: hertz.njit.edu Simple Mail Transfer Service Ready.
• C: HELO pegasus.rutgers.edu
• S: 250
hertz.njit.edu
• C: MAIL FROM: <jverga@pegasus.rutgers.edu>
• S: 250
OK
• C: RCPT TO: <dzt8474@hertz.njit.edu>
• S: 250
OK
Lesson 26 - (23 of 41)
Example SMTP Interaction:
(Continued)
• C: DATA
• S: 354 Start mail input; end with
<CR><LF>.<CR><LF>
• C: …sends body of mail message…
• C: …continues for as many lines as message contains
• C: <CR><LF>.<CR><LF>
• S: 250
OK
• C: QUIT
• S: 221 hertz.njit.edu
Service closing transmission.
Lesson 26 - (24 of 41)
Post Office Protocol (POP):
• Post Office Protocol - Version 3 (POP3):
– Is intended to permit a workstation to
dynamically access a maildrop (mail server) on
a server host in a useful fashion.
– Usually, this means that the POP3 is used to
allow a workstation to retrieve mail that the
server is holding for it.
– Access to a POP3 server requires a username
and password.
Lesson 26 - (25 of 41)
E-Mail Application Program Interfaces:
MAPI
• Messaging Application Program Interface
(MAPI):
– Is a set of messaging functions that helps you
create mail-enabled applications as well as
other message-based applications.
– With MAPI functions, you can easily add
messaging to any Windows application and
may or not include a user interface (a dialog
box).
– It is supported by many vendors (Microsoft).
Lesson 26 - (26 of 41)
E-Mail Application Program Interfaces:
AOCE
• What is AOCE?
– AOCE stands for Apple Open Collaboration
Environment.
– It is a set of technologies that provide for
system-level collaboration and messaging
services.
– The first incarnation of AOCE was in the form
of PowerShare and PowerTalk, the AOCE
server and client, respectively.
– It is a proposed standard interface to e-mail.
Lesson 26 - (27 of 41)
MIME Extensions:
• Multipurpose Internet Mail Extensions (MIME):
– Defined by IETF to allow transmission of nonASCII data via e-mail.
– It allows arbitrary data to be encoded in ASCII
for normal transmission.
– All media types that are sent/received over the
World Wide Web are encoded using different
MIME types.
Lesson 26 - (28 of 41)
MIME Extensions:
822 Mail Header
• Messages sent using MIME encoding include
information that describes the type of data and the
encoding that was used (resides in 822 mail
header).
• 822 Mail Header:
– Version of MIME used.
– Type of data sent.
– Encoding method used.
Lesson 26 - (29 of 41)
Example 822
Mail Message Header:
•
•
•
•
•
From:
dzt@mttec.mt.att.com
To:
jverga@pegasus.rutgers.edu
MIME-Version: 1.0
Content-Type: image/gif
Content-Transfer-Encoding: base64
…data for the image...
Lesson 26 - (30 of 41)
MIME Types and Subtypes:
• All different media encoding done in MIME is
classified as a combination of types and subtypes.
• Each MIME Content-Type must contain two
identifiers:
– Content Type
– Content Subtype
Lesson 26 - (31 of 41)
MIME Types and Subtypes:
• There are seven official (standardized) contenttypes that can appear in a MIME Content-Type
declaration:
– Text - Textual documents.
– Image - A computer generated image.
– Audio - A sound file.
– Video - A motion video recording.
Lesson 26 - (32 of 41)
MIME Types and Subtypes:
– Application - Raw data for a program.
– Multipart - Multiple messages, each having a
separate content type and encoding.
– Message - An entire e-mail message or an
external reference to a message (FTP server and
filename).
Lesson 26 - (33 of 41)
MIME Types and Subtypes:
• An eight type has been proposed and will most
likely be approved to supply a secure method for
transferring encoded data:
– Secure - Data is sent encrypted and in a secure
fashion.
• The push for an official secure MIME contenttype has come about due to the increased interest
in doing business over the WWW.
Lesson 26 - (34 of 41)
S/MIME Content Type:
• What is S/MIME?
– A specification for secure electronic mail.
– S/MIME stands for Secure/Multipurpose
Internet Mail Extensions and was designed to
add security to e-mail messages in MIME
format.
• The security services offered are authentication
(using digital signatures) and privacy (using
encryption).
Lesson 26 - (35 of 41)
S/MIME Content Type:
• Why S/MIME?
– In the growing demand for e-mail security,
S/MIME melds proven cryptographic
constructs with standard e-mail practices.
• Is S/MIME a standard?
– No. Although the S/MIME spec will be
submitted to the IETF for consideration as an
official Internet RFC (standard) when
interoperability tests are complete.
Lesson 26 - (36 of 41)
MIME Types and Subtypes:
• Examples of Content Type/ Subtype:
– image/jpeg
– text/html
– video/mpeg
– audio/x-wav
– application/octet-stream
Lesson 26 - (37 of 41)
MIME Multipart Messages:
• Multipart content type has four subtypes, each of
which adds significant flexibility to MIME
encoding:
– Mixed - allows a single message to contain
multiple independent sub-messages, each of
which is separately encoded (HTML documents
with images).
Lesson 26 - (38 of 41)
MIME Multipart Messages:
– Alternative - a single message can include
multiple representations of the same data type
(ASCII and Rich Text Format).
– Parallel - a single message that includes
subparts which should be viewed together
(video with audio).
– Digest - a single message that can contain a set
of other messages (e-mail messages from a
discussion).
Lesson 26 - (39 of 41)
Questions
E-mail, SMTP, and MIME types
• Why do you think MIME-encoded messages used
as the foundation of WWW technology?
• How does one setup a mail-reflector in there
account to forward e-mail?
• A listserv is a special type of e-mail broadcasting.
Explain.
• What are Mail APIs used for? Give an example.
• What specific MIME type(s) would be used to
send an HTML page with in-line video/audio
Lesson 26 - (40 of 41)
support?
Reference Materials
E-mail, SMTP, and MIME types
• RFC 1651, 1652, 1653: SMTP Services
(http://www.cis.ohio-state.edu/htbin/rfc/rfc1651.html)
• E-Mail API Web Resources:
(http://andrew2.andrew.cmu.edu/cyrus/email/standards-apis.html)
• RFC 1081, 1082: Post Office Protocol - Version 3
(http://www.cis.ohio-state.edu/htbin/rfc/rfc1081.html)
• Writing your own SMTP gateway:
(http://www.vb-online.com/sep95/smtp.html)
• RFC 1521, 1522: MIME Extensions and Headers
(http://www.cis.ohio-state.edu/htbin/rfc/rfc1521.html)
Lesson 26 - (41 of 41)