• This chapter is about e-mail transfer -- it is an example of client-server interaction
  
  

• People can interact with e-mail directly, or
  
  
• processes can communicate via e-mail.
  
  

• Usually a person gets a computer account and a file on the computer is assigned to be the person's electronic mailbox
  
  
• Often the e-mail address has two parts.
  
  
• The first part is often the person's login name.
  
  
• The second part is often the domain name of the computer where the mailbox is located.
  
  
• For example:
  
  jack@wkstn01.big_univ.edu
  
  
• Other formats occur, but most are of the form:
  
  name@domain_name.
  
  
• It is convenient for e-mail transmission software if the second part of the address is the domain name of the receiving computer.
  
  
• Likewise it is convenient if the first part of the address is the user name of the intended recipient (person).
  
  
• When things are set up that way, the e-mail software gets all the information it needs from the address, and administrators on each host are free to assign whatever user names they like -- two hosts can never assign the same e-mail address to different people if the hostname parts are unique to begin with.
  
  

From: The SANS Institute 
Date: Wed Nov 13, 2002  11:54:04  US/Pacific
To: John Sarraille (SD129734) 
Subject: SANS NewsBites Vol. 4 Num. 46

***********************************************************************
SANS NewsBites            November 13, 2002             Vol. 4, Num. 46
***********************************************************************

TOP OF THE NEWS
11 November 2002  CA Law Requires Reporting of Certain Security
                 Breaches
5 & 8 November 2002  Breeders' Cup Investigation Continues
12 November 2002  One Week Left For National Cyber Security Strategy
                  Comments [Please Add Your Voice]
4 November 2002  Financial Sector Cyber Incidents Often Go Unreported

• A typical e-mail message, as above, consists of a header, blank line, and body .
  
  
• Each line of the header begins with a keyword and a colon.
  
  
• Each header is required to have a "To:" line.
  
  
• "New" header keywords don't cause a problem to "old" e-mail software because the software is written to just "pass through" header lines it does not understand. "New" header lines can be used by "new" e-mail software to help add "new" functionality.
  
  

• When you send an e-mail to one or more people, you may want to send a copy of the message to additional people, but not address the e-mail directly to those additional people.
  
  
• For example if your boss tells you to order some supplies, you may send an e-mail to the vendor ordering the supplies and you may send a copy of the message to your boss so s/he will know that you obeyed. This is logical -- you don't want to address the message directly to your boss because the message is an order for merchandise and it is not your intention to order the merchandise from your boss.
  
  
• You use the "Cc:" header line for sending such copies. All people who receive the message see the names on your "To:" list and your "Cc:" list. It is common courtesy to let all your recipients know who has received copies of the message.
  
  
• Rarely, you may find it necessary or expedient to hide such information from your recipients. If you put an address on a "Bcc:" line then a copy of the message will go to that address but there will be nothing in the received header that indicates that the message had a "Bcc:" line, or what address was in the "Bcc:" line.
  
  
• For example, if a co-worker sends you an inappropriate e-mail, you might send that person an e-mail asking him to stop. You might include the e-mail address of your supervisor in a "Bcc:" line, as a way of letting your supervisor know what you are doing. There may be reasons you don't want the offending co-worker to know that you have involved the supervisor.

• Internet e-mail was designed only to transmit printable ASCII characters (plain text).
  
  
• Nowadays e-mail software encodes arbitrary kinds of data as sequences of printable ASCII characters and transmits it.
  
  
• The sender puts lines such as:

  
  MIME-Version: 1.0
  Content-Type: Multipart/Mixed; Boundary=Mime_separator
  

  in the header to tell the sender that MIME conventions are used in the message and that the message consists of different parts separated by the string "Mime_separator."
  
  
• The sender is free to choose any string for a separator. (Of course it will choose something that is not used elsewhere in the body of the message.)
  
  
• The sender can put directives in the message that tell the recipient how to decode each part of the message.
  
  
• Using MIME we can send attachments such as a photograph, word processor document, or spreadsheet.
  
  
• MIME is very flexible: one can add new message types and encoding schemes without any need to change the software that transmits the mail. Everything that is transmitted is encoded as printable ASCII so old e-mail software has no problems -- the MIME message is transmitted exactly the same way as "old-fashioned" plain text.
  
  
• (The glitch with MIME is that often senders and receivers do not use the same e-mail software and so conventions understood by one "end" may not be understood by the other "end." When in doubt just send plain text -- and not as an "attachment." It may not be "fancy" but it usually works because all e-mail software is supposed to understand plain text messages.)
  
  

• A process can send e-mail to a process.
  
  
• For example you might use e-mail software to send a database query to a process, which might then e-mail results back to you.
  
  

• We use e-mail interface software when we compose or read e-mail.
  
  
• Usually there is separate mail transfer (transmission) software that takes care of sending a copy of an e-mail message to each intended recipient.
  
  
• When e-mail is intended for someone on the local machine, the e-mail transfer software has only to append the message to the end of the user's mailbox file.
  
  
• When a recipient is on a remote host, the local e-mail transfer software becomes a client of a server on the remote host and sends the message to the server. That server then appends the message to the local mailbox.
  
  

• SMTP is the (TCP/IP) protocol used in client-server mode by e-mail transfer software.
  
  
• SMPT handles reliable delivery details.
  
  
• SMTP allows sender to ask the receiving host if the intended recipient mailbox exists.
  
  

• If a user on host X sends a message to three users on host Y then the e-mail transfer process only transmits one copy of the message from X to Y. On Y the e-mail transmission software puts a copy of the message in each of the three recipients mailboxes.
  
  
• It would be a waste of network bandwidth to send three copies of the message from X to Y.
  
  

• A process can function as a mail-exploder.
  
  
• You send e-mail to the process, and the process forwards a copy of your message to each member of a large mailing list.
  
  

• Often dedicated e-mail gateways run e-mail exploders because the task can use up a lot of computing resources.
  
  
• Many of the mailing lists involved are public.
  
  

• Many Internet hosts have server processes that manage mailing lists.
  
  
• Clients can create a new list or join or quit a list by sending an e-mail message to such a server.
  
  
• Users get better response from a list-server than humans could provide, an at a much lower cost.
  
  

• An organization like our university can choose to run a mail gateway and give all users an e-mail address of a form like this:
  
  j_sarraille@csustan.edu
  
  
• When mail for "j_sarraille" comes to the machine csustan.edu, an exploder running there could forward the mail to my local e-mail address of
  
  john@ishi.csustan.edu
  
  
• The host csustan.edu would have a database -- a table. The exploder just looks up a name like j_sarraille in the table and reads off the corresponding forwarding address.
  
  
• This is very convenient for senders: to know that everyone here at our campus has an e-mail address of the same form. If I know that someone's name is Jane Smith, then I can immediately guess that I can send her mail at jsmith@csustan.edu.
  
  
• The mail gateway will take care of whatever forwarding task is needed.
  
  
• This scheme makes it very easy to arrange for forwarding e-mail to new addresses. If Jane Smith wants to use a different workstation here, or change her login name then all we have to do is change the entry in the table on the host csustan.edu and the mail will be forwarded to her properly.
  
  

• Often it is not a good idea to choose your personal computer as the host to (directly) receive your e-mail. The mail host has to stay up all the time, running a SMTP server.
  
  
• It may be better to make some workstation your mail host, and to use POP or IMAP to access your e-mail on your personal computer.
  
  
• POP is essentially just a protocol that allows you to download a copy of your mail from your mail host to your personal computer. POP asks you for the password on the mailhost -- to insure that only you have access to your private e-mail.
  
  
• POP does not work well if you want to read your e-mail on more than one host, or if you want to organize your e-mail into folders while working on more than one computer. The basic problem is that POP does not let you organize the mail on the server into folders. All you can do is download the newest mail in the mailbox. If you read mail on more than one client machine then you have to organize the same messages on each client machine separately.
  
  
• IMAP is similar except that it keeps the primary copy of all messages on the mail host and allows you to organize your mail into sets of folders on the mailhost. That is very handy if you want to read your mail from several different personal computers, workstations, and what-have-you. No matter what machine you use to read your mail, you are always working with the same copy of each message and the same set of organized folders.
  
  

• POP and IMAP are popular with users who dialup connections between their personal computer and the Internet.
  
  
• Once connected to the Internet the personal computer user employs POP or IMAP to get access to e-mail.