Figure 23-3 shows how sendmail uses aliases. Programs that read mail, such as /usr/bin/mailx, can have aliases of their own, which are expanded before the message reaches sendmail. The aliases for sendmail can originate from a number of name service sources, such as local files, NIS, or NIS+. The order of the lookup is determined by the nsswitch.conf file. Refer to the nsswitch.conf(4) man page.

Figure 23-3 How sendmail Uses Aliases

sendmail Features

The sendmail program provides the following features.

• sendmail is reliable. The program is designed to correctly deliver every message. No message should ever become completely lost.

• sendmail uses existing software for delivery whenever possible.

• sendmail can be configured to handle complex environments, including multiple connections to a single network type, such as UUCP or Ethernet. sendmail checks the contents of an address as well as its syntax to determine which mailer to use.

• sendmail uses configuration files to control mail configuration instead of requiring that configuration information be compiled into the code.

• Users can maintain their own mailing lists. Additionally, individuals can specify their own forwarding mechanism without modifying the domain-wide alias file, typically located in the domain-wide aliases that are maintained by NIS or NIS+.

• Each user can specify a custom mailer to process incoming mail. The custom mailer can provide functions such as returning a message that reads: "I am on vacation." See the vacation(1) man page for more information.

• sendmail batches addresses to a single host to reduce network traffic.

Figure 23-4 shows how sendmail interacts with the other programs in the mail system.

Figure 23-4 Interaction of sendmail With Other Mail Programs

As shown in Figure 23-4, the user interacts with a mail-generating and mail-sending program. When the mail is submitted, the mail-generating program calls sendmail, which routes the message to the correct mailers. Because some of the senders might be network servers and some of the mailers might be network clients, you can use sendmail as an Internet mail gateway. See Interactions of Mail Programs for a more detailed description of the process.

sendmail Configuration File

A configuration file controls the way that sendmail performs its functions. The configuration file determines the choice of delivery agents, address rewriting rules, and the format of the mail header.

The sendmail program uses the information from the /etc/mail/sendmail.cf file to perform its functions. Each system has a default sendmail.cf file that is installed in the /etc/mail directory. You do not need to edit or change the default configuration file for mail servers or mail clients. The only systems that require a customized configuration file are mail hosts and mail gateways.

The Solaris operating environment provides three default configuration files in the /etc/mail directory.

1. A configuration file that is named main.cf for the system, or systems, you designate as the mail host or a mail gateway.

2. A configuration file that is named subsidiary.cf, which is a duplicate copy of the default sendmail.cf file.

3. A configuration file that is named submit.cf, which is used to run sendmail in mail-submission program mode, instead of daemon mode. For more information, refer to New Configuration File, submit.cf.

The configuration file that you use on a system depends on the role of the system in your mail service.

• For mail clients or mail servers, you do not need to do anything to set up or edit the default configuration file.

• To set up a mail host or mail gateway, copy the main.cf file and rename the file to sendmail.cf in the /etc/mail directory. Then reconfigure the sendmail configuration file to set the relay mailer and relay host parameters that are needed for your mail configuration. For task information, refer to Setting Up Mail Services (Task Map) or Building the sendmail.cf Configuration File (Task) in Chapter 22, Mail Services (Tasks).

The following list describes some configuration parameters that you can change, depending on the requirements of your site.

• Time values, which specify the following information.

  • Read timeouts. Refer to Changes to the Timeout Option.

  • Length of time a message remains undelivered in the queue before the message is returned to the sender. Refer to New Queue Features. For a task map, refer to Administering the Queue Directories (Task Map).

• Delivery modes, which specify how quickly mail is delivered.

• Load limits, which increase efficiency during busy periods by not attempting to deliver large messages, messages to many recipients, and messages to sites that have been down for a long time.

• Log level, which specifies the kinds of problems that are logged.

Mail Alias Files

You can use any of the following files, maps, or tables to maintain aliases.

• .mailrc Aliases

• /etc/mail/aliases File

• NIS aliases Map

• NIS+ mail_aliases Table

Your method of maintaining aliases depends on who uses the alias and who needs to be able to change the alias. Each type of alias has unique format requirements.

If you are looking for task information, refer to Administering Mail Alias Files (Task Map) in Chapter 22, Mail Services (Tasks).

.mailrc Aliases

Aliases that are listed in a .mailrc file are accessible only by the user who owns the file. This restriction enables users to establish an alias file that they control and that is usable only by its owner. Aliases in a .mailrc file adhere to the following format.

alias aliasname value value value ...

aliasname is the name that the user uses when sending mail, and value is a valid email address.

If a user establishes a personal alias for scott that does not match the email address for scott in the name service, mail is routed to the wrong person when people try to reply to mail that is generated by this user. The only workaround is to use any of the other aliasing mechanisms.