Overview

When sending emails from a web application, ecommerce platform, or automated PHP script (e.g., PHPMailer), your messages may be rejected by outbound email filters (such as MailChannels, Gmail, or Outlook) with errors like:

550 5.7.1 [CS] Message blocked

This error frequently occurs when an application script is configured using a raw server IP address (e.g., 192.0.2.1) instead of a fully qualified domain name (FQDN).
 

Why Is the Email Blocked?

Anti-spam filters inspect both the headers and the body content of outgoing emails. Raw IP addresses trigger security flags for two main reasons:

1. Raw IP Links in Message Body: Spammers frequently use raw IP links to obscure destinations. Legitimate email services penalize body links formatted as http://192.0.2.1/verify instead of https://example.com/verify.

2. Raw IP in Message-ID Header: PHP mailing libraries often construct the default Message-ID header using the local host IP (e.g., <uniqueid@192.0.2.1>). Email standards require the domain name after the @ symbol (e.g., <uniqueid@example.com>).
    

How to Resolve the Issue (3 Steps)

Step 1: Map a Subdomain to Your Server IP

If your website or application is hosted on a specific server IP address, create a DNS A Record pointing a subdomain (e.g., app.yourdomain.com) to that IP.

• If using Plesk Hosting DNS: Refer to our guide on How to access your DNS records in your Plesk hosting package to add an A Record (e.g., Host: app, Points to: YOUR_SERVER_IP).

• If using Rebel Domain DNS: Refer to our guide on How do I access the Advanced DNS for my domain to log into your Domain Manager and add an A Record.

Step 2: Update Your Application Settings

In your web application's administration panel or configuration file (config.php, .env, etc.):

1. Locate the Site URL or Domain Name setting.

2. Replace http://YOUR_SERVER_IP with your new subdomain (e.g., [http://app.yourdomain.com](http://app.yourdomain.com)).

3. Save your changes.

This ensures all generated email links and header parameters use your registered domain name rather than the raw IP.

Step 3: Authenticate via SMTP

Do not attempt to run an unauthenticated local mail server. Ensure your application script is configured to send outgoing mail using SMTP Authentication through your official email account (e.g., notifications@yourdomain.com).

Once these updates are complete, outgoing emails will satisfy security filters and deliver normally.