UNIX System Administration Handbook - Evi Nemeth [319]
• Outlook Express from Microsoft,4
also for Windows
Some user agents support a system-wide configuration file that sets defaults for all users. Individual users can override these defaults by setting up a personal configuration file in their home directories. User agents from the Windows and Mac worlds are configured from the application’s UI, although most also support some form of global or automatic configuration for use at large sites.
Table 19.3 shows the features of the various user agents and the locations of their startup files. The locations and even the names of the global configuration files are sometimes vendor-specific; many are in /etc.
Table 19.3 User agent features and configuration files
a. pine calls sendmail by default, but it can also speak SMTP.
b. Not supported in the standard release, but a patch is available.
The SMTP column refers to the way the user agent conveys mail to the transport agent or mail submission agent. A check means that the user agent opens a network connection directly to the transport or submission agent. No check means that the user agent executes the transport or submission agent as a subcommand.
Transport agents
A transport agent must accept mail from a user agent, understand the recipients’ addresses, and somehow get the mail to the correct hosts for delivery. Most transport agents also act as message submission agents for the initial injection of new messages into the mail system. Transport agents speak the Simple Mail Transport Protocol (SMTP) defined in RFC821 or the Extended SMTP protocol (ESMTP) defined in RFCs 1869, 1870, 1891, and 1985.
Several transport agents are available for UNIX (PMDF, Postfix, smail, qmail, exim, and zmailer, among others), but sendmail is the most comprehensive, most flexible, and most widely used (75% at last estimate).
Delivery agents
A delivery agent accepts mail from a transport agent and actually delivers it to the appropriate local recipients. Mail can be delivered to a person, to a mailing list, to a file, or even to a program.
Each type of recipient may require a different agent. /bin/mail is the delivery agent for local users. /bin/sh is the original delivery agent for mail going to a file or to a program. Recent versions of sendmail ship with safer local delivery agents called mail.local and smrsh (pronounced “smursh”). procmail from www.procmail.org can also be used as a local delivery agent; see page 587.
mail.local should not be used on systems such as HP-UX that deliver mail to users’ mailboxes by exploiting chown’s ability to give away files to other users. Older Solaris systems should not use mail.local either, but its use is supported as of Solaris 7.
Message stores
As email grew from servicing the computer science department at a university to servicing sites such as America Online with millions of subscribers, the UNIX filesystem became inadequate as a message store. Searching a directory that contains a million mailboxes is prohibitively expensive.
The message store is the spot on the local machine where email is stored. It used to be the directory /var/spool/mail or /var/mail, with mail being stored in files named after users’ login names. That’s still the standard message store, but ISPs with thousands or millions of email clients are looking to other technologies for the message store (databases, usually).
On systems that use the standard /var/spool/mail or /var/mail store, the mail directory is created during the installation of the operating system. It should have permissions set to mode 775, with group owner mail.
Access agents
Programs such as imapd and spop are access agents for PC, Mac, or UNIX users whose mail is delivered to a UNIX server and then downloaded with the Internet Message Access Protocol (IMAP) or the Post Office Protocol (POP), respectively. IMAP and POP are covered starting on page 549.
Mail submission agents
Another newcomer to the mail arena that was necessitated by high volume