sendmail(8) is the default Mail Transfer Agent (MTA) in FreeBSD. sendmail's job is to accept mail from Mail User Agents (MUA) and deliver it to the appropriate mailer as defined by its configuration file. sendmail can also accept network connections and deliver mail to local mailboxes or deliver it to another program.
sendmail uses the following configuration files:
| Filename | Function |
|---|---|
| /etc/mail/access | sendmail access database file |
| /etc/mail/aliases | Mailbox aliases |
| /etc/mail/local-host-names | Lists of hosts sendmail accepts mail for |
| /etc/mail/mailer.conf | Mailer program configuration |
| /etc/mail/mailertable | Mailer delivery table |
| /etc/mail/sendmail.cf | sendmail master configuration file |
| /etc/mail/virtusertable | Virtual users and domain tables |
The access database defines what host(s) or IP addresses have access to the local mail
server and what kind of access they have. Hosts can be listed as OK, REJECT, RELAY or simply passed to sendmail's
error handling routine with a given mailer error. Hosts that are listed as OK, which is the default, are allowed to send mail to this host as
long as the mail's final destination is the local machine. Hosts that are listed as REJECT are rejected for all mail connections. Hosts that have the
RELAY option for their hostname are allowed to send mail for
any destination through this mail server.
Example 29-1. Configuring the sendmail Access Database
cyberspammer.com 550 We do not accept mail from spammers FREE.STEALTH.MAILER@ 550 We do not accept mail from spammers another.source.of.spam REJECT okay.cyberspammer.com OK 128.32 RELAY
In this example we have five entries. Mail senders that match the left hand side of the table are affected by the action on the right side of the table. The first two examples give an error code to sendmail's error handling routine. The message is printed to the remote host when a mail matches the left hand side of the table. The next entry rejects mail from a specific host on the Internet, another.source.of.spam. The next entry accepts mail connections from a host okay.cyberspammer.com, which is more exact than the cyberspammer.com line above. More specific matches override less exact matches. The last entry allows relaying of electronic mail from hosts with an IP address that begins with 128.32. These hosts would be able to send mail through this mail server that are destined for other mail servers.
When this file is updated, you need to run make in /etc/mail/ to update the database.
The aliases database contains a list of virtual mailboxes that are expanded to other user(s), files, programs or other aliases. Here are a few examples that can be used in /etc/mail/aliases:
Example 29-2. Mail Aliases
root: localuser ftp-bugs: joe,eric,paul bit.bucket: /dev/null procmail: "|/usr/local/bin/procmail"
The file format is simple; the mailbox name on the left side of the colon is
expanded to the target(s) on the right. The first example expands the mailbox
root to the mailbox localuser,
which is then looked up again in the aliases database. If no match is found,
then the message is delivered to the local user localuser. The next example shows a mail list. Mail to the
mailbox ftp-bugs is expanded to the three local mailboxes
joe, eric, and paul. Note that a remote mailbox could be specified as <user@example.com>. The next example shows writing mail
to a file, in this case /dev/null. The last example shows
sending mail to a program, in this case the mail message is written to the
standard input of /usr/local/bin/procmail through a
UNIX® pipe.
When this file is updated, you need to run make in /etc/mail/ to update the database.
This is a list of hostnames sendmail(8) is to accept as the local host name. Place any domains or hosts that sendmail is to be receiving mail for. For example, if this mail server was to accept mail for the domain example.com and the host mail.example.com, its local-host-names might look something like this:
example.com mail.example.com
When this file is updated, sendmail(8) needs to be restarted to read the changes.
sendmail's master configuration file, sendmail.cf controls the overall behavior of sendmail, including everything from rewriting e-mail addresses to printing rejection messages to remote mail servers. Naturally, with such a diverse role, this configuration file is quite complex and its details are a bit out of the scope of this section. Fortunately, this file rarely needs to be changed for standard mail servers.
The master sendmail configuration file can be built from m4(1) macros that define the features and behavior of sendmail. Please see /usr/src/contrib/sendmail/cf/README for some of the details.
When changes to this file are made, sendmail needs to be restarted for the changes to take effect.
The virtusertable maps mail addresses for virtual domains and mailboxes to real mailboxes. These mailboxes can be local, remote, aliases defined in /etc/mail/aliases or files.
Example 29-3. Example Virtual Domain Mail Map
root@example.com root postmaster@example.com postmaster@noc.example.net @example.com joe
In the above example, we have a mapping for a domain example.com. This file is processed in a first match order down the
file. The first item maps <root@example.com> to the
local mailbox root. The next entry maps <postmaster@example.com> to the mailbox postmaster on the host noc.example.net.
Finally, if nothing from example.com has matched so far, it will
match the last mapping, which matches every other mail message addressed to someone at
example.com. This will be mapped to the local mailbox joe.