Pre-setup

Create mailgun flex account at https://www.mailgun.com. Then make an SMTP user and take down these.

username (name@domain.com / postmaster@domain.com)
password
smtp hostname (smtp.eu.mailgun.org / smtp.us.mailgun.org)

Postfix Setup

Firstly install postfix. On the tui that's displayed select internet site and enter your FQDN.

sudo apt-get update && sudo apt-get install postfix

If you don't get an interactive prompt. Fully remove and reinstall postfix.

sudo apt purge postfix && sudo apt-get install postfix

Add the regular user to the postfix group to send mails without auth problems

usermod -a -G postfix <username>

After installation edit postfix config file at /etc/postfix/main.cf. Find suitable settings from the reference[^1]

mydestination = localhost.domain, localhost
relayhost = [<SMTP hostname>]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:<postmaster@domain.com>:<password>
smtp_sasl_security_options = noanonymous
 
smtp_tls_security_level = may
smtpd_tls_security_level = may
smtp_tls_note_starttls_offer = yes
 
# domain mapper
smtp_generic_maps = hash:/etc/postfix/generic

To customize the domain name mails are sent as, a domain mapper must be created for each user. Create a new file /etc/postfix/generic. In it add records for each user. The first part identifies the user in the host server. The second part is what's displayed in the email's from section.

username@hostname sender@domain
someone@zen noreplay@thelonelylands.com

Then to initialize change run

sudo postmap /etc/postfix/generic
sudo service postfix reload

Postfix configs are errorless if the result is none

sudo postfix check
sudo service postfix reload

Further you check runtime logs for errors

service postfix status ==> active(exited)

Finally test it using sendmail

echo "Some text" | sendmail <recipient-email>

Troubleshooting

If you encounter the warning below when checking config. Just act as if you didn't see it. Source (opens in a new tab)

postfix/postfix-script: warning: symlink leaves directory: /etc/postfix/./makedefs.out

The 535 Authentication Error usually implies an error in the data we give. May it be a password, a link or the hostname. Source (opens in a new tab)