Mail Services

A Mail Server’s Function A mail server has basically 4 Functions Accept Mail (From other servers) Send Mail (To a remote server) Accept Mail (From one user to be sent out to another server) Send mail (To end user)

A sample Message POP or IMAP Outgoing mail server SMTP Incoming mail server SMTP User B User A

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POP Post Office Protocol When you read your mail, all of it is immediately downloaded to your computer and no longer maintained on the server.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 Protocol protocol 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? 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 Outgoing Mail Server BLOCKED Authorized user Unauthorized user

Different Unix Mail Servers Sendmail Qmail POSTFIX SMAIL EXIM MMDF ZMAILER

Other Mail Servers Exchange - Microsoft GroupWise - Novell Lotus notes – IBM

The SMTP Protocol HELO hotmail.com bobo@bobo.com 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 QUIT 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: bobo@bobo.com Subject: 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 -0800 220 ESMTP spoken here

A sample SMTP Session helo fred@fred.com mail from: bigelow@rcc.on.ca 250 Requested mail action okay, completed mail from: bigelow@rcc.on.ca rcpt to: rbigelow@hotmail.com data 354 Start mail input; end with <CRLF>.<CRLF>

A sample SMTP Session con’t From: bigelow@rcc.on.ca To: bobo@hotmail.com Subject: 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 messages RETR – Downloads Messages DELE – Deletes a Message