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How to setup smtp server

Published 10/13/2022

Prepare server

  • sudo hostnamectl set-hostname example.com
  • sudo apt install mailutils postfix
    • Choose “internet site”, and type your domain (example.com)

Test postfix

  • mail your-test@yopmail.com -s "Subject"
  • You should receive a mail from debian@example.com

TLS

Generating certificates

  • sudo apt install certbot
  • sudo certbot certonly --standalone --rsa-key-size 4096 --agree-tos --preferred-challenges http -d example.com
    • You might need to kill your running webserver (port 80) to complete the challenge in standalone mode

Giving rights to postfix

  • sudo chown -R root:postfix /etc/letsencrypt/live/example.com

Postfix config

/etc/postfix/main.cf

# TLS parameters
smtpd_tls_cert_file=/etc/letsencrypt/live/example.com/fullchain.pem
smtpd_tls_key_file=/etc/letsencrypt/live/example.com/privkey.pem
smtp_use_tls=yes

SPF and DMARC

  • SPF: v=spf1 ip4:<your_ipv4> ~all
  • DMARC: v=DMARC1;p=none;pct=100;rua=yourmail@example.com;sp=none;aspf=r;

DKIM

SMTPD

Enable submission

  • Uncomment submission’s lines in /etc/postfix/master.cf
  • change smtpd_sasl_type=dovecot by smtpd_sasl_type=cyrus
  • add -o smtpd_sasl_security_options=noanonymous

Install Cyrus SASL

  • sudo apt install sasl2-bin
  • sudo usermod -aG sasl postfix

Configure SASL

  • /etc/default/saslauthd

    • START=yes
    • MECHANISMS="sasldb"
    • OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

  • sudo systemctl restart saslauthd

  • systemctl status saslauthd

    • Make sure it is running with the /var/spool arguments

Add a user

  • sudo saslpasswd2 -c -u example.com user

    • The user does not have to be the user you will send mail with, it is just credentials

  • sudo testsaslauthd -u user -p password -f /var/spool/postfix/var/run/saslauthd/mux

    • Add a space before the command to not keep this in your history
    • Note the custom socket path, it is required

/etc/postfix/sasl/smtpd.conf

pwcheck_method: saslauthd
mech_list: PLAIN LOGIN

sudo postfix reload

Final test

  • Use https://www.mail-tester.com/ to make sure everything is working properly
  • Example code (TS) below, can be ran with npx ts-node test.ts
  • Use real data to test your setup or SpamAssassin will not be happy
import nodemailer from 'nodemailer';

const smtpEndpoint = 'example.com';
const port = 587;
const senderAddress = 'My name <my-address@example.com>';
const smtpUsername = 'user';
const smtpPassword = 'pass';

const transport = nodemailer.createTransport({
	host: smtpEndpoint,
	port: port,
	secure: false,
	auth: { user: smtpUsername, pass: smtpPassword }
});

transport.sendMail({
	from: senderAddress,
	to: 'your-mail-tester-addr',
	subject: 'A real subject',
	text: 'A real body'
});

Additional steps for Outlook/Live

Configure virtual aliases

/etc/postfix/main.cf

virtual_alias_maps = hash:/etc/postfix/virtual

/etc/postfix/virtual

@example.com debian

sudo postmap /etc/postfix/virtual

Sign up to sendersupport

Useful to debug

  • tail -f /var/log/mail.info
  • -v in submission parameters (/etc/postfix/master.cf)

Additional resources