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SMTP Error 521

Category: Other
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Description: <domain> does not accept mail (see rfc1846).

What does this mean?

The recipient mail server does not actually accept and deliver email itself. From this point, emails can either fail or be relayed/forwarded to another mail server. Most commonly, this is shown for relays.

AOL Specific Reference

If you received this error message in your inbox, it means that you tried to send someone at AOL an email message, and AOL’s mail server rejected the connection because it’s seen just a bit too much spam lately from your mail server. Ref: AOL 521.

AOL SMTP Error 521 Example

521 5.2.1 : (CON:B1)

How to fix SMTP Error 521?

  • Check to ensure it is actually failing and not being relayed.
  • If the email is actually failing, contact the recipient mail server to determine the reason for not accepting mail.

For fixing the AOL Error Response, please see: AOL What can I do?