Install msmtp for forwarding system emails on Debian

How to set up msmtp on Debian 11 for forwarding system emails to an SMTP server or email provider (Office 365, Gmail)

• msmtp is light SMTP email client that can be easily configured to forward emails to an SMTP server/email provider
• msmtp supports server profiles that can be used for sending mail to different email providers
• msmtp-mta creates an alias for sendmail; this means it can be used with any system that requires sendmail 

Install msmtp and msmtp-mta

apt-get install msmtp msmtp-mta

Create an aliases file

/etc/aliases is used by sendmail to redirect messages sent to local users/recipients to an email address

sudo nano /etc/aliases

mailer-daemon: email@domain.com
postmaster: email@domain.com
root: email@domain.com
default: email@domain.com 

Rebuild aliases database

newaliases

Configure msmtp

sudo nano /etc/msmtprc

Send email using msmtp without authentication

This example creates a profile called smtp that will be used to send mail to an internal SMTP relay server that doesn't require authentication. The smtp profile is set as the default account.

# Set default values for all accounts
defaults
port 25
tls off
account smtp
host IP-ADDRESS
from EMAIL@DOMAIN.COM
auth off
# Set a default account
account default : smtp
# Aliases file
aliases /etc/aliases 

Send email using msmtp to Office 365

This example creates a profile called o365 that will be used to send mail using Office 365

account o365
host smtp.office365.com
port 587
tls on
tls_starttls  on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
auth on
from EMAIL@DOMAIN.COM
user EMAIL@DOMAIN.COM
password SECRET
# Set a default account
account default : o365
# Aliases file 

Change the permissions of /etc/msmtprc

Change the permissions of /etc/msmtprc so that owner (root) can read/write and group (msmtp) can read the file

sudo chown root:msmtp /etc/msmtprc
sudo chmod 640 /etc/msmtprc

Send a test email

Send a test email to an external address
echo "Test email using msmtp" | msmtp EMAIL@DOMAIN.COM

Send a test email to root
echo "Testing root email alias"| msmtp root

-v print debugging information for troubleshooting
echo "Testing root email alias"| msmtp -v root