Sending Emails via SMTP

msmtp requires a minimal setup for sending emails via SMTP compared to sendmail. Here is a configuration for you to send emails from a web host to an external SMTP server. Prior to doing that, you must check whether there is a clear communication channel between your web host and the SMTP server. You can use Telnet.

Set up msmtp

You are going to set msmtp as an MTA. Hence, you need to remove all other MTAs such as postfix and sendmail:

$ sudo apt-get --purge autoremove postfix sendmail

Install msmtp and related utilities:

$ sudo apt-get install msmtp msmtp-mta mailutils

Configure msmtp:

$ sudo nano /etc/msmtprc

# Set default values for all following accounts.
defaults

# Use the mail submission port 587 instead of the SMTP port 25.
port 587

# Always use TLS.
tls on

# Set a list of trusted CAs for TLS. The default is to use system settings, but
# you can select your own file.
tls_trust_file /etc/ssl/certs/ca-certificates.crt

# The SMTP server
account mx
host mail.mx.example
from admin@mx.example
auth on
user admin@mx.example
password 123456

# Set default account to mx
account default: mx

# Map local users to mail addresses
aliases /etc/aliases

You need to change some of the values above with your own. The msmtp configuration is actually a list of commands which are executed top down. The account command defines the name of the subsequent email account details. You can give it any name. The commands host, from, auth, user and password defines the email account details. You can get more explanation and other options from the msmtp documentation.

Install mailx so that you can use mail:

$ sudo apt-get install bsd-mailx

Set msmtp as MTA:

$ sudo nano /etc/mail.rc

append the following:

set mta=/usr/bin/msmtp

Setup aliases so that the system services such as cronjobs will redirect system user emails to the external SMTP server:

$ sudo nano /etc/aliases

# Send root to John
root: john_doe@mx.example
   
# Send everything else to admin
default: admin@mx.example

$ sudo nano /etc/mail.rc

append:

alias root root<john_doe@mx.example>

Test

$ msmtp -d joe@mx.example <<END
> From: Admin <admin@mx.example>
> To: joe@mx.example
> Subject: test
> 
> test
> END

Note: Some email servers like Gmail insist the From email address. Otherwise, the email will be rejected.

If msmtp hangs at the following:

...
reading recipients from the command line

it could be msmtp is trying to connect to the SMTP server but the port is not open by your web host. msmtp will resolve to IPv6 first. If it failed or timed out then it will resolve to IPv4. Or, if you are using port 465 then ensure the Start TLS option is off in /etc/msmtprc by issuing the tls_starttls off command.

If your web host allows SMTP ports on IPv4 but not on IPv6 then you may need to set timeout to the shortest so that the trials to resolve to Ipv6 will timeout quickly. Issue command timeout 1 in /etc/msmtprc.

Let's send another test email. This time is sending to the system root that should actually go to john_doe@mx.example as defined in the /etc/aliases:

$ mail -s "Hi root" root <<END
> Testing msmtp from ${HOSTNAME} with mail command
> END

Let's send to someone@gmail.com:

$ mail -s "Hi there" someone@gmail.com <<END
> Testing msmtp from ${HOSTNAME} with mail command
> END

Notice the From address was not specified. This is because mail will set it according to the command from as in /etc/msmtprc, i.e. admin@mx.example.

If you want the From address to be the preferred email address of the current system user then you need to create a configuration file in the user home folder ~/.msmtprc. Firstly, msmtp will always try to execute a configuration from ~/.msmtprc. If it does not exist then msmtp will execute /etc/msmtprc.

PHP Mail

PHP uses sendmail to send emails. You need to replace sendmail with msmtp which is sendmail compatible. The following is a typical setup for PHP mail using msmtp. The details depend on your web server. Here we are using PHP7 and Apache2 on Debian 10.

Edit the php.ini file:

$ sudo nano /etc/php/7.3/apache2/php.ini

Find sendmail_path and assign it with the following:

sendmail_path = "/usr/bin/msmtp -t"

Restart apache (or your web server):

$ sudo service apache2 restart

Test sending an email with PHP:

if (mail("your@email.com", "Test email from PHP", "msmtp as sendmail for PHP"))
  echo "Successful";
else
  echo "Failed";

You may need to read the PHP mail documentation on the additional_headers parameter to specify a different From address, or to add CCs.