# Yahoo SMTP Error Codes

> Yahoo Mail's SMTP error/deferral categories (4XX temporary, 5XX permanent, TS* deferrals, PH* content blocks) with causes and recommended actions.

Source: emailmarketing.net — https://emailmarketing.net/learn/providers/yahoo-smtp-error-codes

When Yahoo Mail defers or rejects your mail, its SMTP response tells you whether to retry and what to fix. Yahoo documents these errors (Delivery Status Notifications, DSNs) **by category rather than in an exhaustive table of individual codes**.

Extended tokens such as `TS*` (temporary deferrals) and `PH*` (blocks for content or phishing) appear in the DSN text next to the numeric code. When you contact Yahoo about persistent errors, include the **exact error and diagnostic codes from your logs** in a Sender Support Request.

## Top-level classes

| Class | Codes | Meaning | Retry? |
|---|---|---|---|
| 4XX temporary | 421, 451 | A temporary problem is blocking delivery | Yes, retry later |
| 5XX permanent | 550, 553, 554 | A permanent problem, and the message is refused | **No.** Never retry, and remove addresses that generate 5xx bounces |

### Causes of 4XX (421 and 451) temporary errors

- Yahoo observed unusual traffic patterns from your sending server's IP address.
- The message had characteristics of spam.
- Mail from your server has been generating complaints from Yahoo users.
- Yahoo mail servers were busy at the time of connection.
- Other suspicious behavior triggered **dynamic deprioritization** of your server.
- Your name server was temporarily unavailable.
- A temporary error prevented Yahoo from determining the authentication results for your domain.

Action: review Yahoo's Sender Requirements & Recommendations (see [Yahoo Sender Requirements](https://emailmarketing.net/learn/providers/yahoo-sender-requirements)) and retry later. If you do not administer the mail server, contact its administrator.

### Causes of 5XX (553 and 554) permanent errors

- The recipient address is invalid.
- The message failed authentication against the DMARC or DKIM policy of the sending domain.
- Yahoo does not accept the content, for policy reasons.
- Other suspicious behavior led to permanent rejection of the SMTP connection.
- **The IP address is listed by Spamhaus.** Check https://www.spamhaus.org.

Action: do **not** retry. Consistent 5xx errors signal a widespread problem, and list managers should have a policy for removing addresses that generate 5xx bounces.

## Error categories

| Category | Typical codes | Cause | Recommended action |
|---|---|---|---|
| RFC compliance failures | 4xx or 5xx | Duplicate headers, badly formatted headers, incorrect MIME types, and other violations of RFC 5321 and 5322 | Fix how messages are built so that they comply with the RFCs |
| From domain that does not resolve (RFC 5321 or 5322) | 451 (timeout or SERVFAIL) or 554 (permanent failure) | The domain to the right of the @ in MAIL FROM or in the header From does not appear valid. Yahoo checks that it exists with an **SOA query**, so subdomains used in MAIL FROM or the header From need **SOA records**, not only A or MX records (even wildcard ones) | Make sure every subdomain used in From or MAIL FROM resolves and is covered by an SOA record |
| Excessive user complaints | Deferral or rejection | Mail that generates too many spam votes from Yahoo users | Monitor your reputation (even senders with a good reputation get spam votes). If the error persists for more than 48h, or follows changes to content or policy, review outgoing mail for objectionable content or practices. On shared IP addresses, traffic from other domains may be hurting you, so consider dedicated IP addresses. Check that recipients opted in properly |
| Excessive unsolicited messages | Deferral or rejection | Very high volume from a single IP address, which is typical of unsolicited bulk mail. Often a **new IP address ramped up too fast** | Do not resend until you have reviewed your practices. Warm up IP addresses slowly (see [IP Warm-Up](https://emailmarketing.net/learn/ip-management/ip-warm-up)). Confirm that all recipients asked for the mail. If the error persists for more than 48h, review content and practices |
| Excessive unknown recipients | Rejection | A server sending to many invalid recipients. It may be an **open relay or open proxy**, and Yahoo does not accept mail from these | Secure your servers, remove bouncing addresses from lists, and examine outbound queues for abuse by spammers |
| Authentication failures | Rejection | Failed SPF, DKIM or DMARC verification. Yahoo rejects mail for a DKIM failure when the signing domain publishes a policy requiring all its mail to be DKIM-signed, the domain is in the DKIM `d=` tag, and the message could not be authenticated (a missing or bad signature) | All senders: SPF or DKIM. Bulk senders: SPF, DKIM and DMARC. Senders on Microsoft 365 or Google Workspace: configure DKIM following their guides. Mailing lists (listservs): put the list address in From: and the actual sender in Reply-To:. Forwarded mail: consider ARC |
| Content-based blocks | **PH\*** errors | Content Yahoo does not accept: viruses, phishing, ransomware, other malware, or links to any of these | Examine outbound queues for objectionable content, and secure servers against abuse |
| Recipient does not exist | 554 (for example `554 delivery error: dd This user doesn't have a yahoo.com account (user@yahoo.com) [-5]`) | The Yahoo account does not exist | Never retry. Remove the address from your list, and confirm the address with the recipient directly |
| Message temporarily deferred | **TS\*** errors (421) | Substantial complaints from Yahoo users, objectionable or spam-like content, **poor IP reputation (of the IP address or its /24 subnet)**, or unusual traffic patterns | Retrying later is acceptable. Review outgoing content. Servers that do not send bulk mail (personal, corporate, education or ISP servers) should check their queues for abuse by spammers. If the error continues over an extended period, submit a Sender Support Request with the error and diagnostic codes |
| Resources temporarily unavailable | 4xx | Yahoo mail servers were busy at the time of connection | Usually brief. Retry after a short period |
| Other policy errors | 550, 554 | A **null MX record (RFC 7505)**. A header that is too big (From, Subject, To). A Date out of range. A header value that cannot be decoded (From, Subject). An address that contains unusual or forbidden characters (for example `,$%?{}<>`) | Fix the record, header or address at fault |

## Reading a Yahoo bounce

A Yahoo failure notice (from "MAILER-DAEMON" or "Mail Delivery Subsystem", with a subject such as "Failed Delivery") has two parts: the reason for the bounce, then a copy of the original message. Here is an example of the line reporting what the remote host said:

```
Remote host said: 554 delivery error: dd This user doesn't have a yahoo.com account (testing123@yahoo.com) [-5] - mta123.mail.mud.yahoo.com
```

The most common cause of failure is an invalid recipient address. If the account "doesn't exist", check the spelling. If bounces continue, the account may have been closed or moved.

## Related articles

- [Yahoo Sender Requirements](https://emailmarketing.net/learn/providers/yahoo-sender-requirements), the requirements whose violation causes most of these errors
- [Yahoo Complaint Feedback Loop](https://emailmarketing.net/learn/providers/yahoo-complaint-feedback-loop), to prevent deferrals caused by complaints
- [IP Warm-Up](https://emailmarketing.net/learn/ip-management/ip-warm-up), to avoid "excessive unsolicited messages" errors on new IP addresses