Email marketing is a powerful tool, but it can be incredibly frustrating when your campaigns fail due to delivery issues.
One of the most common culprits is the SMTP Error 550, which indicates that the recipient’s mail server has rejected your email. This error is a permanent delivery failure, meaning the server won’t attempt to resend the message.
In this guide, we’ll explore the causes of SMTP Error 550, its different types, and actionable steps to fix it. By the end, you’ll have the knowledge to troubleshoot and prevent this error, ensuring your emails reach their intended recipients.
What Is SMTP Error 550?
SMTP Error 550 is a status code that signals the recipient’s mail server has rejected your email. This error is often accompanied by a bounce message that provides details about the rejection. Since it’s a permanent failure, the server won’t retry sending the email, making it crucial to resolve the issue before attempting to resend.
Common Types of SMTP Error 550 and Their Causes
SMTP Error 550 can occur for various reasons. Here are the most common types and their causes:
1. 550 High Probability of Spam
This error occurs when the recipient’s server flags your email as spam. Common triggers include:
- Suspicious Content: Missing unsubscribe links, ALL CAPS text, blacklisted URLs, or invalid HTML code.
- Blacklisted IP or Domain: Your IP or domain may be listed on a Realtime Blackhole List (RBL).
- Unauthorized Sender: SPF authentication failed, often due to incorrect DNS records or multiple SPF records.
- Invalid DKIM Signature: The message was altered in transit, or the DKIM record syntax is invalid.
- Failed DMARC Authentication: SPF and DKIM alignment failed, indicating the email may be from an illegal source.
2. 550 This Message was Classified as Spam
This error is triggered when the email content or sending method resembles spam. For example, sending emails via Microsoft Outlook without SMTP authentication can result in this error.
3. 550 5.7.0 Message Size Violation
The email exceeds the size limit set by the recipient’s server. Large attachments or embedded images are common culprits.
4. 550 Denied by Policy
This error occurs when emails are sent to randomly generated addresses, triggering security software to block them.
5. 550 Blocked
This error can result from:
- Improper DNS settings.
- Poor sender reputation.
- User-reported spam.
- Email quotas being exceeded.
6. 550 #5.1.0 Address Rejected
The recipient’s server won’t accept emails from your address, often because it’s blocked.
7. 550 Authentication is Required for Relay
SMTP authentication isn’t enabled, preventing the email from being sent.
How to Fix SMTP Error 550
Most SMTP Error 550 issues stem from problems on the sender’s side. Here’s how to resolve them:
1. Optimize Your Email Content
- Maintain a 60:40 text-to-image ratio.
- Use valid HTML code.
- Avoid ALL CAPS and suspicious attachments.
- Include an unsubscribe link.
2. Remove Your IP or Domain from Blacklists
Request removal from RBLs and address the root cause of the blacklisting.
3. Properly Configure DNS Records
- Ensure your SPF record includes legitimate sources and complies with the 10-DNS lookup limit.
- Enable DKIM authentication.
- Publish a valid DMARC record.
4. Check for Account Compromise
Review your Sent Items for unusual activity, scan for malware, and change your password if necessary.
5. Limit Email Volume
Avoid exceeding daily email limits to protect your domain’s reputation.
6. Verify SMTP Configuration
Enable SMTP authentication and ensure your sending IP or domain is allowed by the server.
7. Improve Sender Reputation
Monitor metrics like spam rate, domain reputation, and delivery errors. Use tools like GlockApps Postmaster to identify and resolve issues.
