On the Internet, sendmail is the most popular UNIX-based implementation of the Simple Mail Transfer Protocol (SMTP) for transmitting e-mail. When a sendmail server receives e-mail, it attempts to deliver the mail to the intended recipient immediately and, if the recipient is not present, it queues messages for later delivery. However, because it does not provide a mailbox facility and for other reasons, other software such as a POP3 or Internet Message Access Protocol server are also needed. Most Internet service providers (ISPs) provide both an SMTP server (such as sendmail) and a POP or IMAP server.
A commercial version of sendmail, called Sendmail, includes a POP3 server and other enhancements to the basic open source sendmail. It also comes in a version that can be installed on a Windows NT operating systems platform.
Sendmail's roots can be traced to the birth of email, occurring in the decade before the birth of ARPANET, the precursor to the Internet. In those days, every user's mailbox was a file that only they had rights to read, and mail applications simply added text to that file. Every user had to wade through their mail file to find any old mail, and reading new mail was a equally difficult. The first actual transfer of a mail message file from one host to another did not take place until 1972, when email began to be moved by FTP over the NCP network protocol. This easier method of communication quickly became popular, even to the point where it made up most of ARPANET's traffic in less than a year.
However, a lack of standardization between competing protocols made email much harder to send from some systems. This continued until the ARPANET standardized on TCP/IP in 1982. Soon after, a new protocol, SMTP, materialized for transporting email messages. These developments, combined with HOSTS files being replaced with DNS, allowed full-featured MTAs to materialize. Sendmail, which grew out of an earlier email delivery system called Delivermail, quickly became the standard as the email began to expand and become widely used.
The sendmail program collects a message from a program like mailx or Mailtool, edits the message header as required by the destination mailer, and calls appropriate mailers to deliver mail or to queue the mail for network transmission. The sendmail program never edits or changes the body of a message. Any changes that it makes to interpret email addresses are made only in the header of the message.
Sendmail is an MTA, or Mail Transfer Agent. Its sole purpose in life is to move mail from one system to another or deliver mail on a single system from one user to another. This sounds simple enough, until you consider the wide variety of systems and network protocols out there and how sendmail manages to communicate with most of them. Most UNIX-derived operating systems come with sendmail out of the box, configured for the most common configuration. In many cases, you can just use it and not worry about how it works. If you have a valid Internet domain name and sendmail is serving just one box properly configured with the fully qualified domain name, sendmail should just magically do its thing.
Where you can run into trouble is if you need to "tweak" sendmail's functionality or if you need to upgrade due to security issues and build from source.
If you think about it, e-mail has been quite a magical revolution in communication. Unlike with the phone, you can still communicate with someone even if you're not both available at the same time. Unlike traditional "Snail Mail", message delivery can happen in minutes; if you're vacationing in Australia, you can still read the e-mail sent to you in Milwaukee, provided you have access to the Internet.
The definitive site for all things sendmail is sendmail.org. This site has links covering where to get sourcecode and documentation, as well as links to other helpful resources. If you have a large organization and want a commercially supported product, the sendmail author has spun off a corporate version, Sendmail, Inc., offering additional features, including graphical configuration, support, and alliances with other key vendors to provide virus scanning and other extras.
The send queue is grouped by the receiving host before transmission to implement message batching. An argument list is built as the scan proceeds. Mail being sent to files is detected during the scan of the send list.
After a connection is established, sendmail makes the per-mailer changes to the header and sends the result to the mailer. If any mail is rejected by the mailer, a flag is set to invoke the return-to-sender function after all delivery is complete.
The sendmail program sends the message to the mailer using one of the same interfaces used to submit a message to sendmail (using the conventional UNIX argument vector or return status, communicating over a pair of UNIX pipes, or using SMTP over a TCP connection). Each copy of the message has a customized header.
Sendmail originated in the early days of the Internet, an era when considerations of security did not play a primary role in the development of network software. Recent versions of Sendmail suffered from a number of security damages that have been corrected over the years.
Sendmail itself connected a certain amount of privilege separation in order to avoid exposure to security issues. As of 2009, current versions of Sendmail, like other modern MTAs, incorporate a number of security improvements and optional features that can be configured to improve security and help prevent abuse.
It's part of your operating system, so you will always get the latest or most stable version and it can serve all local users as well.
As it is used by a broad user base it is well tested and reliable.
You have all the feature sendmail offers (to get a clue, just look into your sendmail.cf.)
It is written in a Compiled Language, so it's fast and can handle high load.
It is important to be aware of what Sendmail is and what it can do for you as opposed to what it is not. In these days of monolithic applications that fulfill multiple roles, you might initially think that Sendmail is the only application you need to run an email server within your organization. Technically, this is true, as Sendmail can spool mail to your users' directories and accepts new email via the command line. But, most users actually require much more than simple email delivery. They usually want to interact with their email using an MUA that uses POP or IMAP to download their messages to their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other applications can work in conjunction with Sendmail and SMTP, but they actually exist for different reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With literally hundreds of different options and rule sets, entire volumes are dedicated to helping explain everything that can be done and how to fix things that go wrong. You should consult the many excellent resources on Sendmail in order to shape it to fit your exact specifications.
However, you should understand what files are installed with Sendmail by default on your system and know how to make basic configuration changes. You should also be aware of how to stop unwanted email (spam) and how to extend Sendmail with the (LDAP).
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