E-Mail Server Essentials
In a Nutshell - CIW Course Section 3, Part B2, Chapter 6
E-mail systems use agents to create, send, store and download e-mail messages.
MTA
The Mail Transfer Agent is responsible for delivering email between hosts. SMTP is an MTA. The Unix sendmail daemon is another example of an MTA.
MDA
The Mail Delivery Agent is responsible for storing email messages until they are collected by the client.
MUA
The Mail User Agent is any client application such as the Unix Pine program or Windows Outlook Express and Netscape Messenger.
Advanced E-Mail Server Configuration
Advanced E-mail configuration topics include: Masquerading, Aliasing and Relaying.
Masquerading allows the SMTP message header to be altered so that it appears to have been sent from a different domain. This can heve legitimate uses within a company with multiple email servers by allowing all corporate email to show a unified address. It can also be misused by spammers who do not wish to be identified.
Aliasing, unlike masquerading, does not alter the message or header at all. It simply allows an abridged name to be associated with an email address for the ease of users.
Relaying allows an SMTP server to receive email for a domain, other than the local domain, and forward this to another SMTP server for further delivery. Most current email servers do not permit this action as it leaves them open to abuse by spam emailers.
SMTP: Connecting Using Telnet
A connection needs to be established to the appropriate port, usually 25, on the SMTP server. My connection was made with the command:
|
telnet xcserver 25 |
Where xcserver is my SMTP server. It is, in fact, a Microsoft Exchange server. Commands that are available include: MAIL FROM, RCPT TO, DATA and QUIT. These are the commands needed to create and send an email. MAIL FROM: SenderEmail, RCPT TO: RecipientEmail set the sender and recipient respectively. The DATA command allows the input of the body text. Type DATA <Enter> then type multiple line of free text. To complete the DATA input type "." (a period) on a line of it's own and <Enter>. The email will now be queued for sending. These commands must be typed in upper case.
POP3: Connecting Using Telnet
As before, a connection is made to the server, this time on port 110. The available commands include:
- USER: allows the client to specify a logon name.
- PASS: allows the client to specify a password.
- LIST: lists any email messages in a mailbox.
- RETR: allows you to read a message
- DELE: deletes the specified message
- QUIT: ends the session.
IMAP
The Internet Message Access Protocol enables email clients to access remote messages as if they were local. IMAP is a more sophisticated version of POP3.
IMAP Functions:
- Allows message access from multiple computers
- Does not rely on less efficient protocol
- Supports online, offline, and disconnected access modes.
- Support concurrent access to shared mailboxes.
- Ensures that the client needs no knowledge of file store format.
- Creates, deletes, and renames mailboxes
- Checks for new messages and deletes messages
LDAP
The Lightweight Directory Access protocol is a method for accessing directory listings. LDAP was designed to adapt a directory system called X.500 for use on modern networks. LDAP does not send or receive email, it is a directory service to facilitate the finding email addresses.