OpenSMTPD

OpenSMTPD is a free mail transfer agent, developed as part of the OpenBSD project. This article builds upon Mail server.

Installation

Install the opensmtpd package.

Configuration

OpenSMTPD is configured in /etc/smtpd/.

Note: Starting with OpenSMTPD version 6.4.0 the configuration file syntax has been completely reworked, breaking compatibility with previous configuration files. For instruction on migrating the configuration to the new syntax see https://poolp.org/posts/2018-05-21/switching-to-opensmtpd-new-config/.

Local mail

This article or section is out of date.

Reason: OpenSMTPD 6.6.3p1 changed the default configuration to use Maildir instead of mbox. (Discuss in Talk:OpenSMTPD#)

To have local mail working, for example for cron mails, it is enough to simply start smtpd.service.

The default configuration of OpenSMTPD is to do local retrieval and delivery of mail, and also relay outgoing mail. See smtpd.conf(5).

Local mail only

To do only local mail, the following is enough:

/etc/smtpd/smtpd.conf

listen on localhost
action "local" mbox alias <aliases>
match for local action "local"

Hybrid : local mail and relay

These two lines in /etc/smtpd/smtpd.conf :

action "local" mbox alias <aliases>
action "relay" relay host "smtp://smtp.foo.bar" mail-from "@foo.bar"
match for local action "local"
match for any action "relay"

configure OpenSMTPD to :

send local email locally, without going through a relay (useful for cron & at mail notifications)

use a relay to send a mail outside of localhost

Simply replace smtp.foo.bar by your ISP mail server, or another server at your convenience.

Relay only

To send all local emails through a relay invoke procmail:

/etc/smtpd/smtpd.conf

action "local" mda "procmail -f -" virtual <aliases>
action "relay" relay host "smtps://label@smtp.foo.bar" auth <secrets> mail-from "@foo.bar"
match for local action "local"
match for any action "relay"

The aliases option is used for the local user mapping, for a simplified mapping you can use virtual aliases with a catch all:

/etc/smtpd/aliases

@ foo@bar

Simple OpenSMTPD/mbox configuration

TLS

To obtain a certificate, see OpenSSL#Usage.

Note: OpenSMTPD has solid defaults, SSLv3 is always disabled and the default ciphers are not known to be insecure. You might still want to test the server as described in Server-side TLS.

Create user accounts

Create a user account on the mail server for each desired mailbox.

# useradd -m -s /bin/bash roger
# useradd -m -s /bin/bash shirley

OpenSMTPD will deliver messages to the user account's mbox file at /var/spool/mail/<username>
Multiple SMTP email addresses can be routed to a given mbox if desired.

Craft a simple smtpd.conf setup

A working configuration can be had in as little as nine lines!

/etc/smtpd/smtpd.conf

pki mx.domain.tld cert         "/etc/smtpd/tls/smtpd.crt"
pki mx.domain.tld key          "/etc/smtpd/tls/smtpd.key"

table creds                    "/etc/smtpd/creds"
table vdoms                    "/etc/smtpd/vdoms"
table vusers                   "/etc/smtpd/vusers"

listen on eth0 tls pki mx.domain.tld
listen on eth0 port 465 smtps pki mx.domain.tld auth <creds>
listen on eth0 port 587 tls-require pki mx.domain.tld auth <creds>

action receive	mbox virtual <vusers>
action send relay

match from any for domain <vdoms> action receive
match for any action send

Create tables

For the domain table file; simply put one domain per line

/etc/smtpd/vdoms

personaldomain.org
businessname.com

For the user table file; list one inbound SMTP email address per line and then map it to an mbox user account name, SMTP email address, or any combination of the two on the right, separated by commas.

/etc/smtpd/vusers

roger@personaldomain.org          roger
newsletters@personaldomain.org    roger,roger.rulz@gmail.com

roger@businessname.com            roger
shirley@businessname.com          shirley
info@businessname.com             roger,shirley
contact@businessname.com          info@businessname.com

For the creds table file; put the user name in the 1st column and the password hash in the 2nd column

Note: Due to an upstream bug (FS#68335), 'smtpctl encrypt' does not work as of opensmtpd 6.7.1p1-6.

/etc/smtpd/creds

roger                              <password hash created using 'smtpctl encrypt' command>
shirley                            <password hash created using 'smtpctl encrypt' command>

Test the configuration

# smtpd -n

If you get a message that says 'configuration OK' - you're ready to rock and roll. If not, work on any configuration errors and try again.

Troubleshooting

Console debugging

If you're having problems with mail delivery, try stopping the smtpd.service and launching the daemon manually with the 'do not daemonize' and 'verbose output' options. Then watch the console for errors.

# smtpd -dv

Subsystem tracing

Add the -T flag to get real-time subsystem tracing

# smtpd -dv -T smtp

Alternately, use the smtpctl trace <subsystem> command if the daemon is already running. The trace output will appear in the console output above as well as the journalctl output for the smtpd.service. For example:

# smtpctl trace expand && smtpctl trace lookup

...will trace both aliases/virtual/forward expansion and user/credentials lookups

Manual Submission port authentication

Encode username and password in base64

# printf '\0username\0password' | base64

Connect to submission port using openssl s_client command

# openssl s_client -host mx.domain.tld -port 587 -starttls smtp

enter ehlo myhostname followed by AUTH PLAIN. Paste in the base64 string from step above after 334 response.

250 HELP
ehlo test.domain.tld
250-mx.hostname.tld Hello test.domain.tld [5.5.5.5], pleased to meet you
250-8BITMIME
250-ENHANCEDSTATUSCODES
250-SIZE 36700160
250-DSN
250-AUTH PLAIN LOGIN
250 HELP
AUTH PLAIN
334 
dXNlcm5hbWUAdXNlcm5hbWUAcGFzc3dvcmQ=
235 2.0.0: Authentication succeeded

"Helo command rejected: need fully-qualified hostname"

When sending email, if you get this kind of messages, set your FQDN in the file /etc/smtpd/mailname. Otherwise, the server name is derived from the local hostname returned by gethostname(3p), either directly if it is a fully qualified domain name, or by retrieving the associated canonical name through getaddrinfo(3).

System users authentication failure

If you are using the system users and the authentication with valid credentials fails, you have to configure PAM:

/etc/pam.d/smtpd

auth required pam_unix.so
account required pam_unix.so
password required pam_unix.so
session required pam_unix.so