Software⏱️ 3 min read📅 2026-06-15

How to Fix: Roundcube SMTP Error (-1): Connection to server failed

Roundcube SMTP error connection to server failed issue resolved by checking server configuration and port numbers.

Quick Answer: Check Roundcube's server configuration, ensure the correct port number is used for SMTP, and verify the server is accessible via the subdomain.

📋 Table of Contents

  1. 🛑 Root Causes of the Error
  2. 🔧 Proven Troubleshooting Steps
  3. ✨ Wrapping Up

The error 'Roundcube SMTP Error (-1): Connection to server failed' occurs when users are unable to send emails via RoundCube, but can receive them successfully. This issue affects users who have manually installed RoundCube on a subdomain and are experiencing problems with email sending.

This error is frustrating for clients as it prevents them from sending emails, which can impact their communication and productivity. In this guide, we will walk you through the possible causes of this error and provide steps to resolve the issue.

🛑 Root Causes of the Error

  • The primary cause of this error is a misconfigured SMTP (Simple Mail Transfer Protocol) settings in RoundCube. When using a subdomain instead of accessing RoundCube via cPanel, it's essential to ensure that the SMTP settings are correctly configured to point to the correct mail server.
  • An alternative reason for this error could be a firewall or proxy configuration issue blocking the connection between RoundCube and the mail server.

🔧 Proven Troubleshooting Steps

Configuring SMTP Settings in RoundCube

  1. Step 1: Step 1: Log in to your RoundCube web interface using the correct username and password. Navigate to the 'Settings' or 'Preferences' section.
  2. Step 2: Step 2: Look for the 'SMTP' settings section and ensure that the 'Server' field is set to the correct mail server IP address or hostname. If you're using a subdomain, make sure it's correctly configured in the DNS settings.
  3. Step 3: Step 3: Verify that the 'Port' field is set to the correct port number (usually 25 for SMTP). You can also configure additional settings such as authentication and encryption methods.

Checking Firewall and Proxy Configuration

  1. Step 1: Step 1: Check your firewall or proxy configuration to ensure that it's not blocking the connection between RoundCube and the mail server. You can do this by checking the firewall logs or proxy settings.
  2. Step 2: Step 2: If you're using a firewall, try whitelisting the port used by RoundCube (usually 80 for HTTP or 443 for HTTPS) to allow incoming traffic.

✨ Wrapping Up

To resolve the 'Roundcube SMTP Error (-1): Connection to server failed' issue, follow the steps outlined in this guide. By configuring your SMTP settings correctly and checking your firewall and proxy configuration, you should be able to send emails successfully via RoundCube.

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