See also Postfix Authenticated Smarthost (Does the same thing without requiring the EPEL repo.)

http://www.linux.com/archive/feature/132006

http://www.squad17.org/node/39

SSMTP is a simple replacement for a full MTA such as Sendmail or Postfix.

• if you already have Postfix installed (RHEL/CentOS), see the link above
• no server daemons
• no running processes
• lightweight
• flexible
• easier than dealing with an MTA
• be sure all you need is a simple outbound sendmail command replacement

apt install ssmtp bsd-mailx

See also Using the EPEL Repository

Install ssmtp:

yum install ssmtp --enablerepo=epel

On one older system, mdadm depended on sendmail, I had to do this first:

rpm -e --nodeps sendmail

For sending system or 'root' mail, you will probably have to manipulate the 'from address' to get SSMTP to send mail through a secure mail server.

vi /etc/ssmtp/revaliases

root:validsender@domain.tld

For SSL connections on SMTPS port 465:

vi /etc/ssmtp/ssmtp.conf

root=validsender@domain.tld      # will be the 'from address'
mailhub=mail.domain.tld:465      # to use SSL on port 465
RewriteDomain=domain.tld         # instead of the host name
#hostname=_HOSTNAME_
#FromLineOverride=YES            # allows setting 'from address' at runtime
UseTLS=YES                       # for SSL
AuthUser=smtpuser
AuthPass=smtppass
#Debug=YES                       # useful for troubleshooting

If you want to use TLS connections on Submission port 587, you will need to add/change:

mailhub=mail.domain.tld:587
#UseTLS=YES
UseSTARTTLS=YES
#TLS_CA_File=/etc/pki/tls/certs/ca-bundle.crt   # Depends on configuration - Not needed on Ubuntu 18.04

If you don't specify a hostname, SSMTP will query the local machine. Since we are authenticating, the hostname doesn't matter too much.

If desired, enable (un-comment) the debug line, then look in /var/log/maillog.

If desired, install the mail command:

yum install mailx

update-alternatives --config mta

echo test | mail -s "testing ssmtp" tester@yourdomain.com

Or with no mail command and no subject line:

echo test | ssmtp -v -f "from@domain.tld" -F "Full Name" tester@yourdomain.com

or

echo test | sendmail -v tester@yourdomain.com

tail -f /var/log/mailog

cat << EOF >> /tmp/test.eml
From: Full Name <validsender@domain.tld>
To: Test User <tester@somedomain.tld>
Subject: For test only!


EOF

ssmtp -t < /tmp/test.eml

Debug=YES

tail -30 /var/log/mail.log

On Centos:

alternatives --config mta