Email is no longer what it used to be. Simple Text Messages. However, the technology behind transmitting emails has not changed greatly since the early days. Mails use the SMTP (simple mail transfer protocol) to transfer messages from one machine to another.
Initially described in RFC 821, the protocol was extended in 1993 (RFC 1425), resulting in the Extended SMTP which provides more commands and new delivery modes.
Message transfer can be broken down into three parts, viz, Mail Transfer Agent (MTA), Mail Delivery Agent (MDA) and Mail User Agent (MUA). The MTA is commonly known as the mail server and it handles the sending of mail to the outside world and listens for incoming messages from the outside world. Sendmail is an MTA. Procmail is an example of an MDA whereas Squirrelmail is an example of MUA.
Sendmail is the world's most popular and one of the earliest open source email servers.
(http://www.sendmail.org) It was originally written before the Internet was standardized and most sites exchanged mails using UUCP protocols. Sendmail at that time provided the much needed features to allow mails to transfer between various systems regardless of their internal characteristics.
So How does it work?
The Sendmail mail server will wait in the background waiting for new messages. When a new message arrives, a child process will be forked to take care of the connection. This mail will then be placed into the Mail Queue and subsequently delivered. If the delivery is not successful, then it will retain the message and keep trying form the mail queue and after a predetermined number of tries and/or duration, it will return a “delivery failed” message to the sender.
Sendmail like many other open source programs has contributors worldwide who are constantly adding new features to the program.
Sendmail is the default mail server on a lot of distributions such as Redhat, Mandrake. Fedora etc and can be installed during the initial installation of the OS.
You can also install it by giving the command
#rpm -Uhv sendmail* where sendmail* is the complete name of the RPM you wish to install.
SpamAssassin
As soon as your mail server is setup, the first visitors to it would in all probability be unwanted messages from spammers, unsolicited advertisers and malaware.
SpamAssassin is just one of the many tools available to help combat spam by setting up rules to tag messages that can have the potential for spam and provide some relief to the users of your mail server.
SpamAssassin uses several ways to determine what is spam:
Checking mail headers – The incoming mail header may contain some well known methods that make an email look like it is coming from a valid user.
Checking Mail Text – The body of an email may contain some standard text that is used by spammers.
Blacklists – There are several blacklist sites that contain list of domains and servers from known sites that relay spam. Using such lists will help you block a lot of spam. However, since spammers keep changing servers and domains, you would have to constantly update your blacklist.
Vipul's Razor is a tool that works with SpamAssassin to check spam signatures that often consist of the exact same message sent thousands of times.
These will not guarantee you complete freedom from spam but will only help you to some extent. Moreover spamAssassin is not a one time solution. You would have to constantly modify your rules and blacklists in order to ensure that not only spam mail gets tagged properly but good mail does not get tagged as spam.
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