Debian11, Server, mSMTP-MTA to send emails alerts
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Every server is not a mail server. There is no need for a full Mail Transport Agent (MTA) on each. A simple relay, such as msmtp is higly sufficient for the server to send email to a smart host. I use it as an exim replacement to provide simple email feature without local delivery on my workstations and servers. This documentation is partially inspired by yakati 1, in French.
Prerequisites
This article only depends on the Generic machine preparation.
Variables
We need to define the SMTP smart host, the mail hub or mail server, we already defined the other variables.
| Variable | Name | Description | Defined in |
|---|---|---|---|
| Hostname | HN | The name of this machine | Configuration variables |
| Domainname | DN | The domain name of this machine | Configuration variables |
| Smarthost | SMTP_HOST | The SMTP hub server FQDN2 | below |
Create
source /root/config.env
cat << EOF >> /root/config.env
export SMTP_HOST="smtp.${DN}" # SMTP server
EOF
vi /root/config.env
Reload
We need to reload the variables in the environment.
source /root/config.env
Installation
This package is very lightweight, let’s install it.
apt-get install -y msmtp-mta
Configuration
It needs to know the FQDN2 of the smart host, ie the official fully featured mail server, which is able to route emails through the internet to the destination. It also needs to know your server FQDN to rewrite local sender names.
cat << EOF >/etc/msmtprc
account default
host ${SMTP_HOST}
auto_from on
maildomain ${HN}.${DN}
EOF
IPTables firewall update
Outgoing network is blocked by default, we need to create a new chain to define what is the default outgoing mail traffic, using SMTP (TCP/25). We include this accept exception to the outgoing traffic on the WAN interface and we reload the firewall configuration.
sed -i 's/^-N WAN_input$/-N SMTP\n-A SMTP -p tcp --dport 25 -j ACCEPT\n\n&/' /etc/iptables/rules.v4
sed -i 's/^-N WAN_output$/&\n-A WAN_output -j SMTP/' /etc/iptables/rules.v4
systemctl restart netfilter-persistent
Materials and links
This documentation is partially inspired by yakati 1, in French.