Electronic Mail
Familiar to most of us
 What are the functions we expect?
– Outgoing mail
• Compose
• send
– compress
– encrypt
– Incoming Mail
•
•
•
•
•
•
read
forward
repl
store (retain) (and organize into folders, search, …)
discard
others????
The X.400 Functional Model
User
P2
UA
UA
MS
P3
(ex: SMTP)
P3
MTA
MTS
MHS
P1
(ex: SMTP)
User
(ex: POP or IMAP)
(ex: SMTP)
AU
MTA
Other
Services
PDAU
Postal
Services
Protocols
 User to the User Agent
– local issue, no need to standardize
 P1: Message Transfer Agent interaction
– concerned with routing and protection
– not interested in message format or content
 P2: User Agent to User Agent
– common packaging and interpretation of labels
 P3: Between the UA or MS and the MTA
– hand over the message for safe delivery
 AU = Access Unit; PDAU = Physical Delivery Access Unit
– non standard delivery, such as e-mail to fax, email to postal mail, etc.
Sending a message
 Use DNS to get IP address of the
destination mail server
 Establish a TCP/IP connection to the server
 (Optional) establish a SSL connection to the
server
 Send header and message
Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549
Routing a message
1. Queries the Directory Server to determine whether the recipient is local or
remote.
2. If the recipient is local, delivers the message, typically placing it in the
message store. Otherwise, proceeds to step 3.
3. If the recipient is remote:
a. Queries DNS to find the MX (mail exchange) servers for the domain.
b. Queries DNS to find the IP address of the remote messaging server.
c. Establishes a TCP/IP connection to the remote messaging server.
d. (Optional) Establishes an SSL connection to the remote messaging
server.
e. Sends the message to the remote messaging server (SMTP-Deliver).
Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549
Retrieving a message
To retrieve a message, the client must know the IP address of the
messaging server, establish a connection to the server, then retrieve the
message using one of the retrieval protocols: POP, IMAP, or HTTP. The
following summarizes the steps the client takes to retrieve a message.
1.Queries DNS to find the IP address of the server.
2.Establishes a TCP/IP connection to the server.
3.(Optional) Establishes an SSL connection to the server.
4.Establishes a POP3, IMAP4, or HTTP connection to the server
to retrieve the message.
Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549
Characteristics
 User Interface
– mail program environment
• hints and clues about what to do next
• reminders of folder names, aliases
– set of individual commands to execute as
needed
• easy to incorporate mail functions into other
programs
 Knowing about success or failure
• failure can occur at any of several places in the mail
delivery system
Mail Interface
 Netscape
 Outlook
 unix mail
 Webmail
 Mail interface programs often provide other
functions, such as calendars and to-do lists
in the same system
Mail system failures
Subject: Re: SIGCSE 2000 sites
Date: 10 Sep 98 09:57:08 -0400
From: ndale@cs.utexas.edu (Nell Dale)
To: <beck>
CC: Boots Cassel <cassel>
My problem with the Arbaretum is its distance from the airport and
town--and there is no public transportation to speak of. We will have to
arrange buses to take people to and from the airport and to and from town.
Taxis are not expensive, but are not used very much here.
Nell
This was actually a failure on our receiver mail
system. It decided that the recipient of the message
did not need to know the e-mail address of anyone
except the sender of the message. It stripped off
everything from the @ on. ( It is fixed now.)
Problem with aliases
.
Subject: Re:VU-UD tickets
Date: Wed, 22 Sep 1999 11:57:22 -0400
From: Dawn_Cassel@company.com
To: wcassel@kemaconsulting.com
CC: Dave@company.com, Kevin@company.com, Mom@company.com,
Dick@company.com, AME@company.com
<message>
Here the problem is that the alias used
locally is shown to the recipients. Those
aliases are of no use in sending replies
to this message
Result Reporting
 Success reporting is usually optional
 Failure reports are more consistent
– Unknown user at the destination site
– Inability to find the destination site
– Can find, but cannot connect to the destination
site
– Badly formed message not processed locally
 Status reporting
– Verbose mode often available to show all the
steps in mail delivery
SMTP
 Workhorse of mail interaction
 Defines message format and transfer
methods
 Uses TCP on port 25
 Send mail to a MTA
 Send mail between MTAs
The List of Reply Codes
211
System status, or system help reply
214
Help message
[Information on how to use the receiver or the
meaning of a particular non-standard command;
t his reply is useful only to the human user]
452 Requested action not taken: insufficient
system storage
500 Syntax error, command unrecognized
[This may include errors such as command
line too long]
501 Syntax error in parameters or argument
502 Command not implemented
220
<domain> Service ready
503
Bad sequence of commands
221
<domain> Service closing transmission channel
504
Command parameter not implemented
250
Requested mail action okay, completed
251
User not local; will forward to <forward-path>
550 Requested action not taken: mailbox
unavailable
[E.g., mailbox not found, no access]
354
Start mail input; end with <CRLF>.<CRLF>
551
421 <domain> Service not available, closing
transmission channel
[This may be a reply to any command if the
service knows it must shut down]
450 Requested mail action not taken: mailbox
unavailable
[E.g., mailbox busy]
User not local; please try <forward-path>
552 Requested mail action aborted: exceeded
storage allocation
553 Requested action not taken: mailbox name
not allowed
[E.g., mailbox syntax incorrect]
554
Transaction failed
451 Requested action aborted: local error in
processing
Ref: http://raddist.rad.com/networks/1998/smtp/smtp.htm#commands2
11 monet:/mnt/a/cassel> mail -v cassel@acm.org
Subject: test message
Checking mail steps
.
EOT
monet:/mnt/a/cassel> cassel@acm.org... Connecting to mail.acm.org. via esmtp.
..
220 mail.acm.org ESMTP Sendmail 8.9.3/8.9.3; Wed, 4 Oct 2000 14:47:43 -0400
From monet
>>> EHLO monet.vill.edu
250-mail.acm.org Hello monet.csc.vill.edu [153.104.202.173], pleased to meet you
250-8BITMIME
250-SIZE 2000000
250-DSN
250-ONEX
250-XUSR
250 HELP
>>> MAIL From:<cassel@monet.vill.edu> SIZE=81
250 <cassel@monet.vill.edu>... Sender ok
>>> RCPT To:<cassel@acm.org>
250 <cassel@acm.org>... Recipient ok
>>> DATA
354 Enter mail, end with "." on a line by itself
>>> .
250 OAA201890 Message accepted for delivery
cassel@acm.org... Sent (OAA201890 Message accepted for delivery)
Closing connection to mail.acm.org.
>>> QUIT
221 mail.acm.org closing connection
From acm.org server
Expanded Message Header
Return-Path:
<cassel@monet.csc.villanova.edu>
Received:
from mail.acm.org ([199.222.69.4]) by skinner.villanova.edu (Netscape Messaging
Server 4.15) with ESMTP id G1X5J300.I88 for <Lillian.Cassel@villanova.edu>;
Wed, 4 Oct 2000 14:47:27 -0400
Received:
from monet.vill.edu (monet.csc.vill.edu [153.104.202.173]) by mail.acm.org
(8.9.3/8.9.3) with ESMTP id OAA201890 for <cassel@acm.org>; Wed, 4 Oct
2000 14:47:43 -0400
Received:
(from cassel@localhost) by monet.vill.edu (8.9.0/8.9.0) id OAA15022 for
cassel@acm.org; Wed, 4 Oct 2000 14:47:44 -0400 (EDT)
Date:
Wed, 4 Oct 2000 14:47:44 -0400 (EDT)
From:
boots cassel <cassel@monet.csc.villanova.edu>
Message-ID:
<200010041847.OAA15022@monet.vill.edu>
To:
cassel@acm.org
Subject:
test message
Addresses
 Local
– same machine
– same local network environment
 remote
– same mail system on a remote host
– different type of mail system
Message Store Protocols
 Client interacting with a mail server
(see RFC 1733)
– offline
• move mail from server to client machine
– online
• keep mail on server and stay connected
– disconnected
• keep mail on server
• download a cache copy and disconnect
• reconnect to synchronize client and server
POP and IMAP
 Post Office Protocol
see RFC 1725 and later revision
– works on local copy on the client machine
(offline access)
– may leave copy on the server, but does not
resynchronize
 Internet Message Access Protocol
see RFC 1730
– allows all three interaction modes
POP commands
USER
Client identifies mailbox to be downloaded
PASS
Password
STAT
Get number of messages and size of mailbox
LIST N
Return size of message N
LAST
Get highest message number accessed
RETR N
Retrieve a full message
TOP N M Retrieve only headers and the first N lines
DELE N
Delete message
QUIT
Release service
NOOP
See if POP server is functioning
RPOP
Insecure authentication
Offline, online, disconnected
 Offline is usual for ISPs.
– They store your messages until you collect them, but don’t provide
long term disk space to keep all your mail for you.
 Online works when the mail is delivered,
read, and stored on the same system
– You login, get your mail, read it, store it, go back to it when you
want to.
 Disconnected
– One copy on your local machine; master copy on a server
– Access available while traveling, from home and office.
– Long distance phone connections are minimized
Mail Summary
 Early in the ARPAnet days, e-mail came to
dominate usage
 With Web use, it remains a major
contributor to traffic on the Internet
 The user interface to the mail system comes
in many varieties
 Behind the scenes, a few message transfer
systems deliver the mail