SMTP clients

This page lists SMTP and SMTPS clients available on OpenWRT, that are able to send email to other email servers.

The table shows all clients ported to OpenWrt:

“smarthost only” means that the program is only capable to send email through a configured “smarthost”, that is, it cannot directly deliver to the destination SMTP server. E.g. it won't make a DNS lookup of the email address for MX record.

msmtp is an SMTP client. In the default mode, it transmits a mail to an SMTP server (for example at a free mail provider) which does the delivery. To use this program with your mail user agent (MUA), create a configuration file with your mail accounts and tell your MUA to call msmtp instead of /usr/sbin/sendmail.

Since msmtp understands standard sendmail options, it can be used in places where sendmail is expected (e.g. PHP code).

• Official site - Sources and documentation
• Arch Wiki

opkg install msmtp-mta

Installing msmtp-mta package will also create necessary sendmail symlink. If you do not need sendmail command (unlikely) then install only msmtp package.

For router configuration, you very likely do not want to receive emails to local mailboxes. Define aliases file /etc/aliases.msmtp to send all local mails to your admin email:

/etc/aliases.msmtp

default: admin@example.com

Place your configuration in /etc/msmtprc. There is an existing default block in the included config file so if you want to call msmtp without specifying an account, then you need to rename the existing default block to something else.

Here is an example configuration using Gmail that works with 2FA and an app password:

/etc/msmtprc

# A system wide configuration file.
# It defines a default account.
# This allows msmtp to be used like /usr/sbin/sendmail.
 
# Set default values.
defaults
aliases /etc/aliases.msmtp
syslog LOG_MAIL
 
# Gmail configuration that works with 2FA and an app password.
# Use TLS on port 465. On this port, TLS starts without STARTTLS.
account gmail
host smtp.gmail.com
port 465
auth on
tls on
tls_starttls off
from_full_name Apartment Router
from home.lab@gmail.com
user home.lab@gmail.com
password abcd efgh ijkl mnop
 
# Select the default account
account default : gmail

Now symlink msmtp to sendmail with ln -s /usr/bin/msmtp /usr/sbin/sendmail - this is not necessary if msmtp-mta was installed instead of msmtp.

Also you may configure env variables EMAIL for From address and SMTPSERVER to specify smarthost.

echo -e "Subject: Test mail\n\nThis is a test \"message\"." | sendmail abcd
echo -e "Subject: Test mail\n\nThis is a test \"message\"." | sendmail -f "<something@your-domain.tld>" "<recipient@destination.tld>"

Note that the sendmail command (/usr/sbin/sendmail) is a symlink to /usr/bin/msmtp.

Use logread to check for the following log entry:

Mon Oct 14 23:35:55 2024 mail.info msmtp: host=smtp.gmail.com tls=on auth=on user=******@gmail.com from=******@gmail.com recipients=admin@example.com mailsize=181 smtpstatus=250 smtpmsg='250 2.0.0 OK  1728948955 b640f23e62c3d-c9d29717ba4su6358366z.33 - gsmtp' exitcode=EX_OK

A secure, effective and simple way of getting mail off a system to your mail hub. It contains no suid-binaries or other dangerous things - no mail spool to poke around in, and no daemons running in the background. mail is simply forwarded to the configured mailhost. Extremely easy configuration.

Official site

opkg install ssmtp

ssmtp expects its two configuration files named /etc/ssmtp/revaliases and /etc/ssmtp/ssmtp.conf. Both are self-explaining:

/etc/ssmtp/ssmtp.conf

root=arnold@gmx.net
mailhub=mail.gmx.net:465
rewriteDomain=gmx.net
hostname=gmx.net
FromLineOverride=YES
UseTLS=YES
#UseSTARTTLS=YES

/etc/ssmtp/revaliases

# Format: local_account:outgoing_address:mailhub
root:arnold@gmx.net:mail.gmx.net:465

To use the program, with SMTP auth:

cat /etc/banner | ssmtp -vvv -auultranerd@universum.tb -ap123password456 someguy@gmx.net

The BusyBox sendmail is a smallest possible implementation but it must be compiled. It works only in smarthost mode. TLS is not supported but can be used with openssl s_client.

• Documentation
• Sources

Enable the applet during compilation in menuconfig: BusyBox options, Mail and sendmail. Additionally you may want to enable makemime applet that helps to send files with attachments.

You need to configure SMTPHOST env variable with the smarthost SMTP server to use. Instead of the variable you may use an option -S server[:port] to specify the SMTP server.

You may also need SMTP_ANTISPAM_DELAY env to bypass the Greylisting antispam if it enabled for submission.

Usage:

sendmail [-tv] [-f SENDER] [-amLOGIN 4<user_pass.txt | -auUSER -apPASS]
		[-w SECS] [-H 'PROG ARGS' | -S HOST] [RECIPIENT_EMAIL]...

Read email from stdin and send it
Standard options:
	-t		Read additional recipients from message body
	-f SENDER	For use in MAIL FROM:<sender>. Can be empty string
			Default: -auUSER, or username of current UID
	-o OPTIONS	Various options. -oi implied, others are ignored
	-i		-oi synonym, implied and ignored

Busybox specific options:
	-v		Verbose
	-w SECS		Network timeout
	-H 'PROG ARGS'	Run connection helper. Examples:
		openssl s_client -quiet -tls1 -starttls smtp -connect smtp.gmail.com:25
		openssl s_client -quiet -tls1 -connect smtp.gmail.com:465
			$SMTP_ANTISPAM_DELAY: seconds to wait after helper connect
	-S HOST[:PORT]	Server (default $SMTPHOST or 127.0.0.1)
	-amLOGIN	Log in using AUTH LOGIN
	-amPLAIN	or AUTH PLAIN
			(-amCRAM-MD5 not supported)
	-auUSER		Username for AUTH
	-apPASS 	Password for AUTH

If no -a options are given, authentication is not done.
If -amLOGIN is given but no -au/-ap, user/password is read from fd #4.
Other options are silently ignored; -oi is implied.
Use makemime to create emails with attachments.

Mailsend is a simple command line program to send mail via SMTP protocol.

Being quite lightweight and not requiring any configuration, mailsend is ideal for sending mails in shell scripts.

https://github.com/muquit/mailsend - Sources and issues

Depending on whether you want SSL support or not (actually, the size of libopenssl will probably be the decisive factor), install one of the two versions:

opkg install mailsend
opkg install mailsend-nossl

Simple usage:

mailsend -f root@openwrt -t foo@example.com -smtp smtp.example.com -sub "My subject" -msg-body /tmp/body

For advanced usage (MIME attachments, authentication, BCC, etc), see:

mailsend -h
mailsend -example

mini_sendmail reads its standard input up to an end-of-file and sends a copy of the message found there to all of the addresses listed. The message is sent by connecting to a local SMTP server. This means mini_sendmail can be used to send email from inside a chroot(2) area.

Official site

opkg install mini-sendmail

On Chaos Calmer, the package is no longer available, but the version from Barrier Breaker still works (AA & BB versions as installed below work in AA):

opkg install http://downloads.openwrt.org/barrier_breaker/14.07/ar71xx/generic/packages/oldpackages/mini-sendmail_1.3.6-4_ar71xx.ipk

mini_sendmail -ssmtp.mail.yahoo.com -p465 -t foo@example.com < input_file
 
usage: mini_sendmail [-f<name>] [-t] [-s<server>] [-p<port>] [-T<timeout>] [-v] [address ...]

Note that there must not be a space between the option and the value; e.g. “-p 465” is incorrect.

Several sample configurations found on the internet failed, but as of 160413, this worked

echo -e 'From: valid@email.com\r\nSubject: Test Subject\r\n\r\nTesting\r\n.' | mini_sendmail -fvalid@email.com -smail.brighthouse.com toAddr@gmail.com

smtp.gmail.com failed; omitting -f failed; omitting “From: ” resulted in transmission, but no sender shown.

This is a very small package if you can find an smtp server which works and you can configure the command so that the server accepts it.