E-mail, SMTP, and MIME Types
LESSON 26

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.
• 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.

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).

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
• 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).

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).
• 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")

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)
• 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 e-mail messages to automatically be forwarded to another specified e-mail account.
• 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).

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).
• 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.

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.
• TCP/IP mail standard has two main parts:
  • Specifies the format for mail messages and their headers.
  • Specifies the details of electronic mail exchange.

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.
• 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

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.
• It provides readable text transcripts of client-server interactions in the mail header and integer values for commands:
  • MAIL FROM
  • RCPT TO
  • HELO
  • 250 OK
  • QUIT

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@megahertz.njit.edu>
S:	250	OK
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.

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.

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).

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.

MIME Extensions:

• Multipurpose Internet Mail Extensions (MIME):
  • Defined by IETF to allow transmission of non-ASCII 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.

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.

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...

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
• There are seven official (standardized) content-types 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.
  • 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).
• 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 content-type has come about due to the increased interest in doing business over the WWW.

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).
• 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.

MIME Types and Subtypes:

• Examples of Content Type / Subtype:
  • image/jpeg
  • text/html
  • video/mpeg
  • audio/x-wav
  • application/octet-stream

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).
  • 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).

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1995 Illusion Industries Inc.