Send emails by local applications only

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It is good if your server can talk to you. Maybe not literally, but it’s good if it can send you emails with logs or issues. It’s also nice to allow your website script located on server to send some emails, to users or administrators. You do not always need a fully configured mail server for this. All you need is a simple MTA for internal communication.

Send Only SMTP

There is a lot of mail server software. I will use one of the most popular, which is Postfix. In this article you will also find two alternatives (SSMTP and MSMTP) for SMTP solution which allow your server to send emails.

Postfix

Here are steps to configure Postfix as a send-only SMTP server.

Install postfix and mail utilities.

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sudo apt install postfix
sudo apt install mailutils

During installation of Postfix you will be asked to choose mail configuration and domain. Choose Internal Site and enter the fully qualified name of your domain, fqdn.example.com. This configuration allow Postfix to process requests to send emails only from the server on which it is running, i.e. from localhost.

Now edit configuration.

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sudo nano /etc/postfix/main.cf

Change the line that reads inet_interfaces = all to inet_interfaces = loopback-only

Also modify mydestination

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mydestination = $myhostname, localhost.$your_domain, $your_domain

That’s all. Now restart Postfix using command:

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sudo systemctl restart postfix

It’s worth to edit aliases:

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sudo nano /etc/aliases

and add email for users or services, example below.

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root: your_email_address1
default: your_email_address2
postmaster: your_email_address3

Save file and run:

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sudo newaliase

Sending test email

To send email use command:

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echo "This is the body of the email" | mail -s "This is the subject line" email_address

Hardening Postfix

In addition, since we are already in the Postfix subject it’s worth to mention how to make your mail server more secure. Here are some tips.

Make sure the Postfix is running with non-root account:

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ps aux | grep postfix | grep -v '^root'

Change permissions and ownership on the destinations below:

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chmod 755 /etc/postfix
chmod 644 /etc/postfix/*.cf
chmod 755 /etc/postfix/postfix-script*
chmod 755 /var/spool/postfix
chown root:root /var/log/mail*
chmod 600 /var/log/mail*

Edit /etc/postfix/main.cf and add make the following changes:

Configure Trusted Networks, for example:

Configure the SMTP server to masquerade outgoing emails as coming from your DNS domain, for example:

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myorigin = example.com

Configure the SMTP domain destination, for example:

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mydomain = example.com

Configure to which SMTP domains to relay messages to, for example:

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relay_domains = example.com

Configure SMTP Greeting Banner:

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smtpd_banner = $myhostname

Limit Denial of Service Attacks:

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default_process_limit = 100
smtpd_client_connection_count_limit = 10
smtpd_client_connection_rate_limit = 30
queue_minfree = 20971520
header_size_limit = 51200
message_size_limit = 10485760
smtpd_recipient_limit = 100

Restart the Postfix daemon:

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sudo service postfix restart

Make it more private

You can hide some information from header of emails sent by your server. To do that, create file 

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sudo nano /etc/postfix/header_checks

and add lines:

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/^Received:/			        IGNORE
/^X-Originating-IP:/            IGNORE
/^X-Mailer:/                    IGNORE
/^Mime-Version:/                IGNORE
/^User-Agent:/                  IGNORE

Now edit Postfix configuration

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sudo nano /etc/postfix/main.cf

and add at the end

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mime_header_checks = regexp:/etc/postfix/header_checks
header_checks = regexp:/etc/postfix/header_checks

Rebuild the hash table:

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postmap /etc/postfix/header_checks

and reload the postfix configuration:

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sudo service postfix restart

SSMTP

SSMTP is a program which delivers email from a local computer to a configured mailhost. It is not a mail server and does not receive mail, expand aliases or manage a queue. One of its primary uses is for forwarding automated email (like system alerts) off your machine and to an external email address.

This is good alternative to Postfix solution and configuration is very easy.

Install software:

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sudo apt install ssmtp

Edit ssmtp configuration:

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sudo nano /etc/ssmtp/ssmtp.conf

Here is example for Gmail account:

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mailhub=smtp.gmail.com:587
rewriteDomain=
UseSTARTTLS=YES
AuthUser=username@gmail.com
AuthPass=your_gmail_password
FromLineOverride=YES

Edit revaliases configuration:

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sudo nano /etc/ssmtp/revaliases

Add:

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root:username@gmail.com:smtp.gmail.com:587

Send test email:

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echo "This is the body of the email" | mail -s "This is the subject line" email_address

To allow PHP mail use this solution you need to make a change in your php.ini configuration file. It’s located in various places depends on distribution and PHP version. Below is and example for PHP7.2 FPM

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sudo nano /etc/php/7.2/fpm/php.ini

change sendmail_path:

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sendmail_path = /usr/sbin/ssmtp -i

MSMTP

In the default mode, it transmits a mail to an SMTP server (for example at a free mail provider) which takes care of further delivery. So doing same thing like SSMTP but is constantly developed (SSMTP is not maintained anymore) and offer more:

  • Sendmail compatible interface (command line options and exit codes)
  • Support for multiple accounts
  • TLS/SSL support including client certificates
  • Many authentication methods
  • Support for Internationalized Domain Names (IDN)
  • Fast SMTP implementation using command pipelining
  • DSN (Delivery Status Notification) support
  • SOCKS proxy support

Lets do it, Install software:

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sudo apt-get install msmtp

Edit msmtp configuration:

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sudo nano /etc/msmtprc

Here is example for Gmail account:

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defaults
tls on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile /var/log/msmtp.log
account gmail
host smtp.gmail.com
port 587
auth on
user username@gmail.com
password your_gmail_password
from username@gmail.com

account default : gmail

Change permission:

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sudo chmod 0644 /etc/msmtprc

Create log file and add permission:

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sudo touch /var/log/msmtp.log
sudo chmod 0777 /var/log/msmtp.log

Send test email using msmtp:

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echo -e "Subject: This is the subject line\r\n\r\nThis is the body of the email" | msmtp -t test@test.com

you can choose from which account message should be sent:

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echo -e "Subject: This is the subject line\r\n\r\nThis is the body of the email" | msmtp --account=gmail-t test@test.com

send message from file:

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cat name.file | msmtp

debug option

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echo -e "Subject: This is the subject line\r\n\r\nThis is the body of the email" | msmtp -t test@test.com -d

debug to file:

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echo -e "Subject: This is the subject line\r\n\r\nThis is the body of the email" | msmtp -t test@test.com -d > test_log

Once everything is working fine, link msmtp as sendmail. You can use ready solution by installing:

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sudo apt install msmtp-mta

or create link manually

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sudo ln -s /usr/bin/msmtp /usr/lib/sendmail
sudo ln -s /usr/bin/msmtp /usr/bin/sendmail
sudo ln -s /usr/bin/msmtp /usr/sbin/sendmail

thanks to that you can send emails using mail command:

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echo "This is the body of the email" | mail -s "This is the subject line" email_address

For PHP mail function is the same story like with SSMTP. Edit your php.ini file:

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sudo nano /etc/php/7.2/fpm/php.ini

and change sendmail_path:

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

Other scripts

For Wordpress you can use for example plug-in EASY WP SMTP. Other popular scripts also have some plug-ins or already implemented solution to send email directly from website script.

That’s all for today. Let me know in comments if you have some suggestions or improvements.