SMTP Error 511 is a permanent or "5xx" error code returned by a mail server, indicating the rejection of an email message due to its inability to relay or deliver the message to the intended recipient. This error denotes that the server encounters difficulties in routing the email to the specified recipient address.

What are the cases covered in SMTP Error 511?

SMTP Error 511 in phpmailer Instances & Examples:

• Case 1: "511 5.1.1 recipient@example.com: Invalid recipient address. Check recipient details."
• Case 2: "511 5.4.2 Routing issue. Unable to relay the message to recipient@example.com."
• Case 3: "511 5.7.3 DNS problem. Email routing to recipient@example.com is currently unavailable."

What’s Causing This SMTP Error 511 In Your Servers? (in pointers)

SMTP 511 error in phpmailer can occur due to:

• Invalid recipient address: The recipient's email address provided in the "RCPT TO" command or the "To:" field of the email message is incorrect, improperly formatted, or does not exist.
• Routing issues: The server may be experiencing problems with its email routing, which prevent it from correctly directing email messages to their intended recipients.
• DNS issues: Domain Name System (DNS) problems or incorrect DNS settings can also lead to SMTP error 511, making it difficult for the server to resolve recipient domains.

How to Resolve SMTP Error 511 - Step-by-Step Solution

To address SMTP Error 511, follow these steps:

• Verify recipient address: Ensure that the recipient's email address is spelled correctly, is complete, and includes the correct domain name. Correct any typographical errors or formatting issues.
• Investigate routing issues: If the error is related to routing problems, you may need to contact your email administrator or hosting provider to resolve the issue.
• DNS issues: If DNS problems are the root cause, work with your DNS provider or administrator to correct any DNS misconfigurations or issues affecting email routing.

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