Install Postfix on your VPS

Postfix is a mail transfer agent, the program that sends and receives email between servers using SMTP. This guide installs Postfix on Ubuntu 24.04 in a send-only configuration, which is the safest choice for application notifications, and sends a first test message.

Before you begin

A VPS running Ubuntu 24.04 LTS, with SSH access as a sudo user (this guide uses alex).
A hostname for the server, such as mail.example.com, with a DNS A record pointing at the VPS IP 203.0.113.10.
Understanding of how email is routed. If not, read How email works: SMTP, IMAP and MX first.

Step 1: Set the server's hostname

Mail servers identify themselves by name during delivery, so the hostname should match the DNS name you created. Set it, then confirm:

sudo hostnamectl set-hostname mail.example.com
hostname -f

mail.example.com

Tip: Receiving servers also check the reverse DNS (PTR) record for your IP. Ask LightServe support by chat or email to set the reverse DNS for your address to mail.example.com.

Step 2: Install Postfix

Install Postfix along with bsd-mailx, which provides the mail command for sending test messages. A setup screen appears:

sudo apt update
sudo apt install -y postfix bsd-mailx

General type of mail configuration. Choose Internet Site. This lets Postfix send mail directly to other servers.
System mail name. Enter example.com, your domain. Addresses will be built from it.

Step 3: Configure Postfix for sending only

Postfix's main settings live in /etc/postfix/main.cf. The postconf -e command edits one setting at a time. Start by making Postfix listen only on the loopback address. Applications on the server can still send through it, but the internet cannot use it to relay spam.

sudo postconf -e "myhostname = mail.example.com"
sudo postconf -e "inet_interfaces = loopback-only"
sudo systemctl restart postfix

Review the settings that now differ from the defaults:

sudo postconf -n

mailbox_size_limit = 0
myhostname = mail.example.com
mydestination = $myhostname, example.com, localhost.localdomain, localhost
inet_interfaces = loopback-only
myorigin = /etc/mailname
...

Careful: Do not set inet_interfaces = all and open port 25 unless you intend to receive mail for your domain. An open relay is quickly abused by spammers and can get your IP address blocklisted.

Step 4: Send a test message

The mail command reads the message body from standard input. The pipe symbol | sends the output of echo into mail, and -s sets the subject line. Replace the recipient with an address you can check:

echo "This is a test message from my VPS." | mail -s "Postfix test" alex@example.org

Watch the delivery log. Postfix logs through the systemd journal on Ubuntu, so read it with journalctl:

sudo journalctl -u postfix -n 30 --no-pager

postfix/smtp[...]: ... to=<alex@example.org>, ... status=sent (250 ...)

A line containing status=sent means the receiving server accepted the message. Check that the queue is empty:

mailq

Mail queue is empty

Step 5: Publish SPF and DMARC records

Without these records, many receiving servers will mark your messages as spam. Add two TXT records in your DNS. If your domain is managed in Lantern DNS, use the DNS editor there.

Name: example.com
Type: TXT
Value: v=spf1 ip4:203.0.113.10 -all

Name: _dmarc.example.com
Type: TXT
Value: v=DMARC1; p=none; rua=mailto:postmaster@example.com

The SPF record says only this server may send mail for the domain. DMARC starts with p=none, which reports problems without rejecting mail, so you can watch results before tightening the policy. DKIM signing is a separate step, which you can add later.

Troubleshooting

"Relay access denied" or the message never leaves. The sender must be on the server itself. With loopback-only, only local programs can send through Postfix.
status=deferred with "Connection timed out". Your provider may be blocking outbound port 25. Ask LightServe support by chat or email.
The test message lands in spam. Check the SPF and DMARC records, the reverse DNS record, and the message headers. A brand-new IP address often has a poor reputation at first.
Postfix will not start. Run sudo postfix check to find configuration errors, then read sudo journalctl -u postfix -n 50 --no-pager.

Next steps

For mail that your server should receive or that users should read, you need an IMAP service such as Dovecot, plus a DKIM signer. Add those when you have a clear need, and keep the server send-only until then.