The sendmail program allows each user to have a :include:-style list to customize the receipt of personal mail. That file (actually a possible sequence of files) is defined by the ForwardPath option (ForwardPath). Traditionally, that file is located in a user's home directory. We use the C-shell notation ~ to indicate user home directories, so we will compactly refer to this file as ~/.forward.
 Prior to V8 sendmail the ~/.forward file could live only in the user's home directory and had to be called .forward.
If a recipient address selects a delivery agent with the F=w flag set (F=w), that address is considered the address of a local user whose ~/.forward file can be processed. If the user part of that address contains a backslash, sendmail disallows further processing, and the message is handed to the local delivery agent's P= program for delivery to the mail-spooling directory. If a backslash is absent, sendmail tries to read that user's ~/.forward file.
If all the .forward files listed in the ForwardPath option (ForwardPath) cannot be read, their absence is silently ignored. This is how sendmail behaves when those files don't exist. Users often choose not to have ~/.forward files. But problems can arise when users' home directories are remotely mounted. If the user's home directory is temporarily absent (as it would be if an NFS server is down), or if a user has no home directory, sendmail syslog(3)s the following error message and falls back to the other directories in its ForwardPath option:
forward: no home
If there are no further directories to fall back to, the missing home is considered a temporary error, and the message is queued for a later delivery attempt.
V8 sendmail temporarily transforms itself into the user before trying to read the ~/.forward file. This is done so that reads will work across NFS. If sendmail cannot read the ~/.forward file (because it is not allowed to), it silently ignores that file.
 This is supported only under operating systems that properly support seteuid(3) or setreuid(3) (USESETEUID).
Before reading the ~/.forward file, sendmail checks to see whether it is a "safe" fileone that is owned by the user or root, has the read permission bit set for the owner, and is writable only by root or the owner. If the ~/.forward file is not safe, sendmail logs a warning and ignores the file.
If sendmail can find and read the ~/.forward file and if that file is safe, sendmail opens the file for reading and gathers a list of recipients from it. Internally, the ~/.forward file is exactly the same as a :include: file. Each line of text in it can contain one or more recipient addresses. Recipient addresses can be email addresses, the names of files onto which the message should be appended, the names of programs through which to pipe the message, or :include: files.
Beginning with V8 sendmail, ~/.forward files can contain comments (lines that begin with a # character). Other versions of sendmail treat comment lines as addresses and bounce mail that is seemingly addressed to #.
The traditional use of the ~/.forward file, as its name implies, is to forward mail to another site. Unfortunately, as users move from machine to machine, they can leave behind a series of ~/.forward files, each of which points to the next machine in a chain. As machine names change and as old machines are retired, the links in this chain can be broken. One common consequence is a bounced mail message ("host unknown") with a dozen or so Received: (Received:) header lines.
As the mail administrator, you should beware of the ~/.forward files of users at your site. If any contain offsite addresses, you should periodically use the SMTP expn command to examine them. For example, consider a local user whose ~/.forward contains the following line:
 Under old versions of sendmail the vrfy and expn commands are interchangeable. Under V8 sendmail and other, modern SMTP servers, the two commands differ.
This causes all local mail for the user to be forwarded to the host remote.domain for delivery there. The validity of that address can be checked with nslookup and telnet(1) at port 25 and the SMTP expn command:
 In place of specifying port 25, you can use either mail or smtp. These are more mnemonic and easier to remember (although we "old-timers" tend to still use 25).
% ns -q=mx remote.domain Address: 18.104.22.168 remote.domain preference = 0, mail exchanger = mail.remote.domain remote.domain preference = 10, mail exchanger = mx.another.domain % telnet mail.remote.domain 25 Trying 22.214.171.124 ... Connected to mail.remote.domain. Escape character is '^]'. 220 mail.remote.domain Sendmail 8.6.13/8.6.13 ready at Sat, 18 Aug 2001 09:48:09 - 0600 (MDT) 220 ESMTP spoken here expn user 250 <email@example.com> quit 221 remote.domain closing connection Connection closed by foreign host. %
This shows that the user is known at remote.site but also shows that mail will be forwarded (yet again) from there to another.site. By repeating this process, you will eventually find the site at which the user's mail will be delivered. Depending on your site's policies, you can either correct the user's ~/.forward file or have the user correct it. It should contain the address of the host where that user's mail will ultimately be delivered.
But beware that the world of email is becoming less friendly for the well-intentioned administrator. Because EXPN can be used to harvest addresses for spam lists, it is more and more frequently turned off. If you connect to a site with EXPN turned off, you will see an error such as the following, instead of the forwarding address you need:
502 5.7.0 Sorry, we do not allow this operation
If EXPN fails, try finger(1) in its place, which also might fail (another illustration of the harm caused by spam email).
Because ~/.forward files are under user control, the administrator occasionally needs to break loops caused by improper use of those files. To illustrate, consider a user who wishes to have mail delivered on two different machines (call them machines A and B). On machine A the user creates a ~/.forward file such as this:
Then, on machine B the user creates this ~/.forward file:
The intention is that the backslashed name (\user) will cause local delivery and the second address in each will forward a copy of the message to the other machine. Unfortunately, this causes mail to go back and forth between the two machines (delivering and forwarding at each) until the mail is finally bounced with the error message "too many hops."
On the machine that the administrator controls, a fix to this looping is to temporarily edit the aliases database and insert an alias for the offending user, such as this:
This causes mail for user to be delivered locally and that user's ~/.forward file to be ignored. After the user has corrected the offending ~/.forward files, this alias can be removed.
The ~/.forward file can contain the names of files onto which mail is to be appended. Such filenames must begin with a slash character that cannot be quoted. For example, if a user wishes to keep a backup copy of incoming mail:
the first line (\user) tells sendmail to deliver directly to the user's mail spool file using the local delivery agent. The second line tells sendmail to append a copy of the mail message to the file specified (in.backup).
Note that, prior to V8, sendmail did no file locking, so writing files by way of the ~/.forward file was not recommended. Beginning with V8, however, sendmail locks those files during writing, so such use of the ~/.forward file is now OK.
If the SafeFileEnvironment option (SafeFileEnvironment) is set, the user should be advised to specify the path of that safe directory:
\user /arch/bob.backup here /arch was specified by the SafeFileEnvironment option
When the SafeFileEnvironment option is used, the cooperation of the system administration might be needed if users are to have the capability of saving mail to files via the ~/.forward file.
The ~/.forward file can contain the names of programs to run. A program name is indicated by a leading pipe (|) character, which might or might not be quoted (Section 12.2.3). For example, a user might be away on a trip and want mail to be handled by the vacation(1) program:
\user, "|/usr/ucb/vacation user"
Recall that prefixing a local address with a backslash tells sendmail to skip additional alias transformations. For \user this causes sendmail to deliver the message (via the local delivery agent) directly to the user's spool mail box.
The quotes around the vacation program are necessary to prevent the program and its single argument (user) from being viewed as two separate addresses. The vacation program is run with the command-line argument user, and the mail message is given to it via its standard input.
Beginning with V8 sendmail, a user must have a valid shell to run programs from the ~/.forward file and to write files via the ~/.forward file. See _PATH... for a description of this process and for methods to circumvent it at the system level.
Because sendmail sorts all addresses and deletes duplicates before delivering to any of them, it is important that programs in ~/.forward files be unique. Consider a program that doesn't take an argument and suppose that two users both specified that program in their ~/.forward files:
user 1 \user1, "|/bin/notify" user 2 \user2, "|/bin/notify"
Prior to V8 sendmail, when mail was sent to both user1 and user2, the address /bin/notify appeared twice in the list of addresses. The sendmail program eliminated what seems to be a duplicate, and one of the two users did not have the program run.
 V8 sendmail uses the owner of the ~/.forward file in addition to the program name when comparing.
If a program requires no arguments (as opposed to ignoring them), the ~/.forward program specifications can be made unique by including a shell comment:
user 1 \user1, "|/bin/notify #user1" user 2 \user2, "|/bin/notify #user2"
Rather than expecting users to write home-grown programs for use in ~/.forward files, offer them any or all of the publicly available alternatives. The most common are listed below.
The procmail(1) program, originally written by Stephen R. van den Berg and currently maintained by Philip Guenther, is purported to be the most reliable of the delivery programs. It can sort incoming mail into separate folders and files, run programs, preprocess mail (filtering out unwanted mail), and selectively forward mail elsewhere. It can function as a substitute for the local delivery agent or handle mail delivery for the individual user. The procmail program (as recommended in its manual) is typically used in the ~/.forward file like this:
"|exec /usr/local/bin/procmail #user"
Note that procmail does not accept a username as a command-line argument. Because of this, a dummy shell comment is needed for pre-V8 versions of sendmail to make the address unique. The procmail program is available from http://www.procmail.org/.
The slocal program, distributed with the mh distribution, is useful for sorting incoming mail into separate files and folders. It can be used with both Unix-style mail files and with mh-style mail directory folders. The slocal program (as recommended in its manual) is typically used in the ~/.forward file like this:
"| /usr/local/lib/mh/slocal -user user"
The disposition of mail is controlled using a companion file called ~/.maildelivery.
Normally, a program in the user's ~/.forward file is executed with the Bourne shell:
Mprog, P=/bin/sh, F=lsDFMeuP, S=10, R=20, A=sh -c $u the Bourne shell
One drawback to using the Bourne shell to run programs is that it exits with a value of 1 when the program cannot be executed. When sendmail sees the exit value 1, it bounces the mail message.
There will be times when bouncing a mail message because the program could not execute is not desirable. For example, consider the following ~/.forward file:
"| /usr/local/lib/slocal -user george"
If the directory /usr/local/lib is unavailable (perhaps because a file server is down or because an automounter failed), the mail message should be queued rather than bounced. To arrange for requeuing of the message on failure, users should be encouraged to construct their ~/.forward files like this:
"| /usr/local/lib/slocal -user george || exit 75"
Here, the || tells the Bourne shell to perform what follows (the exit 75) if the preceding program could not be executed or if the program exited because of an error. The exit value 75 is special, in that it tells sendmail to queue the message for later delivery rather than to bounce it.