Introduction

If you want or need to set up your mail server to send all outgoing mail through a specific server that will handle message delivery, you need to configure an SMTP relay, also known as a smarthost.

Prerequisites

  • You need to know the hostname of the SMTP server that will relay your outgoing mail — in this case, it's relay.stacksmtp.com
  • This guide assumes that SMTP authentication is required on the relay server
  • This article is written specifically for Plesk with either Postfix or Qmail as the MTA
  • You need to have Plesk Admin login credentials ready

Step 1: Log in to Plesk

Go to:
https://x.x.x.x:8443
Replace x.x.x.x with your actual server IP address.

Log in using your Plesk admin credentials.

Step 2: Open Mail Server Settings

Navigate to:
Tools & Settings > Mail Server Settings

Step 3: Enable SMTP Relay (Smarthost)

  1. Under Outgoing mail mode, select:
    ✅ Send from the specified IP addresses
  2. Then check:
    ✅ Send outgoing mail via external SMTP server

Step 4: Enter SMTP Relay Server Details

Fill in the external SMTP server settings:

SMTP server: relay.stacksmtp.com
Port: 587
Use SMTP authentication: Yes
Username: user
Password: password
Encrypted connection: Required

Note: Ensure port 25 , 587 is open in your firewall and that your SMTP credentials are correct. TLS encryption is required for secure transmission.

Step 5: Save Changes

Click OK or Apply at the bottom of the page to save the configuration. Plesk will automatically apply the changes to the underlying MTA.

Conclusion

Congratulations, you have successfully configured an SMTP relay (smarthost) with authentication and encryption on your Plesk server. All outgoing mail will now be routed securely through the specified relay server.

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