Port 25 and sending mail from the server

Summary

Port 25 (SMTP) may be blocked by default on the server as an anti-spam measure. To send mail from the server, configure a mail server and request port unblocking if needed.

Applies to:
✔ VPS
✔ Dedicated servers

Why port 25 is blocked

Outgoing port 25 blocking prevents spam from new servers. This is standard hosting provider practice.

Check port 25 availability

telnet gmail-smtp-in.l.google.com 25

If the connection hangs or fails, outgoing port 25 is blocked.

ss -tlnp | grep :25

How to request unblocking

Open a support ticket with:

• Server IP address
• Purpose (own mail server, sending notifications)
• Domain from which email will be sent

Preparation before sending mail

Before requesting unblock, configure:

1. PTR record for the IP (request via support)
2. SPF record in DNS
3. DKIM signature
4. DMARC record

Alternative: sending via SMTP relay

If port 25 is blocked and unblocking is not needed, use an external SMTP service (port 587 or 465). Configure Postfix as a relay:

relayhost = [smtp.provider.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt

systemctl restart postfix

Verification

telnet gmail-smtp-in.l.google.com 25

echo "Test" | mail -s "Test" your@email.com

Port 25 unblocking may take up to 24 hours. If the port is still unavailable after unblocking, open a follow-up support ticket.