SMTP error 551 signifies your email was permanently rejected (5xx) and couldn't reach the recipient's address due to recipient information issues (phpmailer, jenkins). This often indicates a non-existent recipient, invalid domain, unauthorized recipient, or content filtering violations.

What are the cases covered in SMTP Error 551?

Common scenarios triggering SMTP Error 551:

1. Nonexistent Recipient: The recipient's email address ("RCPT TO" or "To:") is invalid and doesn't correspond to a real account or mailbox (phpmailer, jenkins).
2. Invalid Recipient Domain: The recipient's email domain (e.g., example.com) is non-existent, expired, or has DNS (Domain Name System) problems.
3. Unauthorized Recipient: The recipient's domain or address cannot receive your email, or it's blocked by their server for policy reasons.
4. Content Filtering: The email content might contain prohibited elements like spam, malware, or violations of the recipient server's policies.

What’s Causing This SMTP Error 551 In Your Servers?

Potential causes of SMTP Error 551:

• Incorrect Recipient Address: Double-check that the recipient's email address is accurate, complete, and belongs to a valid account or mailbox (phpmailer, jenkins).
• Recipient Domain Issues: Verify that the recipient's email domain is functioning correctly, has no DNS issues, and isn't expired.
• Contact Recipient Administrators: If the recipient's domain has issues, their administrators might need to intervene to resolve technical problems or adjust email acceptance policies.
• Review Email Content: Analyze the email for potential policy violations like spammy content, excessive attachments, or malicious links.

How to Resolve SMTP Error 551 - Step-by-Step Solution

1. Verify Recipient Address: Ensure the recipient's email address is accurate, complete, and belongs to a valid account or mailbox (phpmailer, jenkins).
2. Check Recipient Domain: Confirm that the recipient's email domain is functioning correctly, has no DNS issues, and isn't expired.
3. Revise Email Content: Address any potential policy violations in the email message, such as removing spammy content or harmful attachments, to comply with the recipient server's policies.
4. Contact Recipient Administrators (if applicable): If the issue lies with the recipient's domain, contact their administrators for assistance.

SMTP Error 551 Examples

• "551 5.1.1 recipient@example.com: Recipient address does not exist."
• "551 5.4.5 recipient@example.com: Domain name not found. Check recipient domain."
• "551 5.7.0 recipient@example.com: Unauthorized recipient. Email blocked due to policy reasons."
• "551 5.1.2 Content filtering detected prohibited content in the email message. Delivery denied."

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