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554 5.7.9 error from Yahoo: what it means

Yahoo's 554 5.7.9 means the message wasn't accepted for policy reasons, typically authentication: the sending domain isn't using DMARC, or fails it.

Sender problemYou are the problem: your domain, IP, authentication or content. Fix that before sending more.

What 554 5.7.9 means

Yahoo554 5.7.9 Message not accepted for policy reasons

Yahoo's help ties it to DMARC: mail from a domain without DMARC, or auto-forwarded mail that now looks like it came from an unauthorized server.

List problem or sender problem?

Sender problem. It's your domain's authentication, or how forwarded mail from it looks to Yahoo. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 554 5.7.9

  1. Set up SPF, DKIM and DMARC for your sending domain, per Yahoo's sender requirements.
  2. If the mail was forwarded, check the raw headers for X-Yahoo-Forwarded and the original domain's p=reject policy.
  3. Don't resend it unchanged: Yahoo treats 5xx codes as permanent.

In SendingOS, 554 5.7.9 shows on the conversation with its reason and code. If bounces like this pile up on one domain, a domain health rule can pause that domain's inboxes before the whole campaign suffers.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

550 5.7.26550 5.7.509

Sources: Yahoo help: 554 5.7.9 · Yahoo: SMTP error codes