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SMTP Error 521: Does Not Accept Mail

SMTP error 521 indicates that the server does not accept mail. This error occurs when the mail server explicitly refuses to accept mail, typically because it's not configured as a mail receiving server.

What Does 521 Mean?

The SMTP reply code 521 follows the standard SMTP response format:

  • 5 = Permanent failure (permanent negative completion reply)
  • 21 = Does not accept mail

When you receive a 521 error, it means the server you're trying to send mail to does not accept incoming mail. This is a permanent error that indicates the server is not configured to receive mail.

Error Type

  • Type: Permanent failure (hard error)
  • Category: Server configuration
  • Action Required: Use a different mail server or contact administrator

Common Causes

  1. Not a Mail Server: The server is not configured as a mail receiving server
  2. Outbound Only: The server only sends mail and doesn't receive it
  3. Relay Server: The server is configured as a relay only, not a destination
  4. Server Misconfiguration: The server is misconfigured and cannot accept mail
  5. Security Policy: Security policies prevent the server from accepting mail
  6. Service Disabled: The mail receiving service is disabled
  7. Wrong Server: Attempting to send to the wrong server

How to Resolve

For Email Senders

  1. Verify Server: Verify that you're sending to the correct mail server
  2. Check MX Records: Check MX records to find the correct mail server
  3. Contact Administrator: Contact the server administrator to enable mail receiving
  4. Use Different Server: Use a different mail server that accepts mail
  5. Check Configuration: Verify your email client configuration

For Developers

  1. MX Record Lookup: Always use MX record lookup to find mail servers
  2. Error Handling: Handle 521 errors as permanent failures
  3. Server Validation: Validate that servers accept mail before sending
  4. Fallback Servers: Implement fallback mechanisms for multiple MX records
  5. Logging: Log 521 errors to track servers that don't accept mail

Related SMTP Errors

Examples

Example Error Message

521 Does not accept mail

Example with Enhanced Status Code

521 5.7.1 Does not accept mail

Common Scenarios

  • Outbound Only Server: "521 This server does not accept mail"
  • Relay Server: "521 Relay server does not accept mail"
  • Misconfigured Server: "521 Mail service not available"

Best Practices

  1. Use MX Records: Always use MX record lookup to find mail servers
  2. Handle as Permanent: Treat 521 errors as permanent failures
  3. Verify Servers: Verify servers accept mail before sending
  4. Error Handling: Implement proper error handling for 521 errors
  5. Logging: Log 521 errors to track problematic servers
  6. Contact Administrators: Contact server administrators if 521 errors persist
  7. Documentation: Document servers that don't accept mail
  8. Monitoring: Monitor 521 error rates to identify configuration issues