Mail Administration Guide

Semantics of Mailer Descriptions

Each mailer has an internal name. It can be arbitrary, except that the names local and prog must be defined first and second respectively. Rule set 0 resolves names to this mailer name (and a host and user name).

Give the path name of the mailer in the P field. If this mailer will be accessed by way of a TCP connection, use the string [TCP] instead.

Define the mailer flags in the F field. Specify an f or r flag to pass the name of the sender as an f or r flag respectively. To conform mailers that give errors under some circumstances, these flags are passed only if they were passed to sendmail. In some cases, you might be able to specify f$g in the argv template. If the mailer must be called as root, and sendmail is running setuid to root, use the S flag; it will not reset the user ID before calling the mailer. If this mailer is local (that is, will perform final delivery rather than another network hop), use the flag. Quoted characters (backslashes and double quotation marks) can be stripped from names if the s flag is specified; if it is not specified, they are passed through. If the mailer is capable of sending to more than one user on the same host in a single transaction, use the m flag. If this flag is on, then the argv template containing $u is repeated for each unique user on a given host. The e flag marks the mailer as being "expensive," and sendmail defers connection until a queue run. Note that the c configuration option must also be set.

The C flag is a useful case. It applies to the mailer from which the message is sent, rather than the mailer where the message is received. If set, the domain specification of the sender (that is, the @host.domain part) is saved and appended to any names in the message that do not already contain a domain specification. For example, a message in this form:

From: eric@jupiter
To: joe@saturn, sam

is modified to:

From: eric@jupiter
To: joe@saturn, sam@ganymede

if and only if the C flag is defined in the mailer corresponding to eric@jupiter.

The S and R fields in the mailer description are per-mailer rewriting sets to be applied to sender and recipient names respectively. These are applied after the sending domain is appended and the general rewriting sets (number 1 or 2) are applied, but before the output rewrite (rule set 4) is applied. A typical use is to append the current domain to names that do not already have a domain. For example, a header in this form:

From: eric@host

might be changed to:

From: eric@host.colorado.edu

or:

From: saturn!eric

depending on the domain into which it is being shipped. These sets can also be used to do special-purpose output rewriting in cooperation with rule set 4.

Table 3-13 includes additional flags that you might use in the configuration file.

Table 3-13 Additional Flags for the Mailer Description


Field Name	Use
`E`	Defines the string to use as an end-of-line indication. A string containing `return` and `newline` is the default (if using TCP; otherwise just a `newline` indicates end-of-line. You can use the `print` backslash escapes (`/r`, `/n` `/f`, `/b`).
`A`	Specifies an argument vector template. It may have embedded spaces. The template is macro-expanded before being passed to the mailer. Useful macros include `$h`, the host name resolved by rule set 0, and `$u`, the user name (or names) resolved. If there is no argument with a `$u` macro in it, `sendmail` uses SMTP to communicate with the mailer. If the path name for this mailer is TCP, use the argument vector: `TCP $h [` `port` `],` where `port` is the optional port number to connect to.
`L`	Specifies the maximum length of the `$u` macro passed to the mailer. To make UUCP mail more efficient, you can use the `L` field with the `m` flag to send mail to multiple recipients with one call to the mailer, while avoiding mailer limitations on argument length. `$u` always expands to at least one recipient even if that recipient exceeds the `L=` limit.

For example, the specification:

Mlocal, P=/bin/mail, F=flsSDFMmnP, S=10, R=20, A=mail -d $u
Mprog,  P=/bin/sh,   F=lsDFMeuP,  S=10, R=20, A=sh -c $u

specifies a mailer for local delivery and a mailer for Ethernet delivery. The first is called local, is located in the file /bin/mail, takes an F flag, does local delivery, strips quotes from names, and delivers mail to multiple users at once. It applies rule set 10 to sender names in the message and applies rule set 20 to recipient names. The argument vector to send to a message is the word mail, the word d, and words containing the name of the receiving user. If you are inserting an r flag is inserted, place it between the words mail and d.

The second mailer is called ether. It is connected through TCP and can handle multiple users at once. It defers connections and appends any domain from the sender name to any receiver name without a domain; it processes sender names by rule set 11 and recipient names by rule set 21. Messages passed through this mailer have a 100,000-byte limit.