An SMTP error 521, encountered within contexts like PHPMailer or Jenkins, indicates that the recipient's mail server has refused to accept the message due to authentication failure or inadequate credentials. This implies that the sender's email application or server couldn't provide valid authentication data, or the provided credentials lacked the necessary authority for relaying the email.

What are the cases covered in SMTP Error 521?

1. Case 1: "521 5.1.2 recipient@example.com: Host or domain does not accept mail. Please verify the recipient's email server settings."
2. Case 2: "521 5.2.1 recipient@example.com: Service temporarily unavailable. Please try again later."
3. Case 3: "521 5.7.0 sender@example.com: Sender domain does not have valid SPF records. Email rejected for security reasons."
4. Case 4: "521 5.7.1 recipient@example.com: Recipient address rejected. Domain does not exist."

Causes of SMTP Error 521:

• SMTP error 521 is frequently observed in mail relays, which serve the purpose of relaying emails to remote mail servers without directly managing delivery.
• The occurrence of this error signals a rejection at the relay level, implying possible complications associated with relaying messages.

Solution to Resolve SMTP Error 521:

• Validate Email Delivery: Start by verifying if the emails sent are effectively delivered through the email relay system. This step helps ascertain whether the problem lies within the relay system itself.
• Communicate with Recipients: In case recipients indicate that they haven't received the emails, it's essential to investigate why the recipient's mail server is rejecting them. This can be accomplished by contacting the administrators of the mail server to gain insights into the reasons for rejection.

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