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Mail Services
A Mail Server’s Function
A mail server has basically 4 Functions
1. Accept Mail (From other servers)2. Send Mail (To a remote server)3. Accept Mail (From one user to be
sent out to another server)4. Send mail (To end user)
A sample Message
Incoming mail server
Outgoing mail server
User A User B
SMTPSMTP POP or
IMAP
The Messaging process
When a message is sent to a mail server it uses SMTP (Simple Mail Transfer Protocol) to send the message.SMTP uses port 25 on the accepting mail server.When the message arrives at the destination server it is then downloaded by a mail client using either POP3 or IMAP protocols
POP
Post Office Protocol When you read your mail, all of it is immediately downloaded to your computer and no longer maintained on the server.
IMAP
Internet Message Access Protocolprotocol in which e-mail is received and held for you by your Internet server. Your e-mail client can view just the heading and the sender of the letter and then decide whether to download the mail
Features of IMAP
You can also create and manipulate folders or mailboxes on the server, delete messages, or search for certain parts or an entire note. IMAP requires continual access to the server during the time that you are working with your mail.
IMAP Vs POP
IMAP can be thought of as a remote file server. POP can be thought of as a "store-and-forward" service.
Common Email Port Addresses
110 pop3
25 smtp
143 imap
Relaying Messages
Relaying is the process of sending or forwarding the message.Most mail servers will restrict where the forward messages from this is to reduce spamming.This is accomplished by setting a number of rules stating who is authorized and who isn’t.
What makes a user authorized?
Outgoing Mail Server
Authorized user
Unauthorized user
•IP address
•Login/password
•The sender’s domain
•The sender’s email address
•The reply to address matches the sender’s address
•The receiver's address/domain
BLOCKED
Different Unix Mail Servers
SendmailQmailPOSTFIXSMAILEXIMMMDFZMAILER
Other Mail Servers
Exchange - MicrosoftGroupWise - NovellLotus notes – IBM
The SMTP Protocol
HELO hotmail.com [email protected] Greet the mail server. Used once per
session - at the beginning of the session.
The SMTP Protocol
MAIL FROM: <from> Announce who the sender is. Used
once per mail, before specifying any recipients for each mail, or after a RSET.
The SMTP Protocol
RCPT TO: <rcpt> Announce who the mail is to. Multiple
recipients are allowed, each must have its own RCPT TO: entered immediately after a MAIL FROM:
The SMTP Protocol
DATA Starts mail entry mode. Everything
entered on the lines following DATA is treated as the body of the message and is sent to the recipients. The DATA terminates with a . (period) on a line by itself. A mail may be queued or sent immediately when the . is entered. It cannot however be reset at this stage.
The SMTP Protocol
RSET Reset the state of the current transaction. The MAIL FROM: and RCPT TO: for the current transaction are cleared.
QUIT
The SMTP Protocol
DATA
From: me@mymachine To: [email protected]: A test message
This is gonna be like a small info bulletin on how... ... .
A sample SMTP Session
homer:~$ telnet mx06.hotmail.com 25
Trying 64.4.55.7...Connected to mx06.hotmail.com.Escape character is '^]'.220-HotMail (NO UCE) ESMTP server ready
at Thu, 14 Feb 2002 05:52:19 -0800220 ESMTP spoken here
A sample SMTP Session
helo [email protected] Requested mail action okay, completed
mail from: [email protected] Requested mail action okay, completed
rcpt to: [email protected] Requested mail action okay, completed
data354 Start mail input; end with
<CRLF>.<CRLF>
A sample SMTP Session con’t
From: [email protected]: [email protected]: Hello
This is a test.
250 Requested mail action okay, completed
quit
The POP Protocol
telnet popserver.ca 110+OK QPOP (version 2.53) at popserver.com starting. <1106.1075384691@popserver>USER username+OK Password required for hacker.Pass password
POP Protocol
LIST _shows messagesRETR – Downloads MessagesDELE – Deletes a Message
