Published May 1, 2026 · Last verified September 5, 2026 · Maintained by TamingDNS

↩️ Yahoo / AOL SMTP Error Code Directory

Yahoo Mail and AOL (both operated by Yahoo Inc.) embed proprietary identifiers in square brackets inside standard SMTP responses. You will see codes like [TSS02] or [TS06] mixed in with the usual SMTP text. This directory covers the standard codes with Yahoo-specific behaviour, and what the bracketed identifiers are actually observed alongside.

Yahoo does not publish these identifiers

Yahoo's own error-codes page lists causes rather than codes, and frames temporary rejections as 421 or 451 and permanent ones as 553 or 554. Every granular per-bracket meaning in circulation, including anything you read below, is inferred from observed bounces rather than documented, and in the wild [TSS02], [TSS04] and [TSS05] all arrive with the same generic deferral sentence. Treat the bracket as a reference number for a conversation with Yahoo. The sentence beside it is the payload.

Provider

Yahoo Mail / AOL Mail / Yahoo Inc.

Key Identifiers

[TSS] · [TS] · mta5.am0.yahoodns.net

MX Hostname Pattern

*.yahoodns.net · *.aol.com

How to Identify a Yahoo / AOL Bounce

Yahoo bounces appear with the Remote-MTA containing yahoodns.net or aol.com. The diagnostic text includes a bracketed code like [TSS02] inside the human-readable rejection reason.

421 4.7.0 [TSS02] Messages from 198.51.100.1 temporarily deferred due to user
complaints or because the IP/domain is in the unverified category. Please make
sure that your email is RFC-5321 compliant. ec=ax9 [16]

The [TSS02] bracket identifies which internal Yahoo rule fired, but Yahoo does not publish what each one means, and the sentence beside it is the same generic deferral text used with other brackets. ec=ax9 [16] is an internal transaction identifier and can be ignored.

Standard Codes with Yahoo-Specific Behaviour

These standard SMTP codes have documented Yahoo-specific behaviour or responses.

🔴 550 5.1.1 CRITICAL

Mailbox Does Not Exist

Yahoo returns: "No such user here." This is final. The address does not exist on their system.

Full explanation & fix →

Yahoo Proprietary [TSS] and [TS] Codes

These bracketed codes appear inside Yahoo's SMTP rejection text. They provide more specific reasons than the outer SMTP code alone.

🟡 [TSS02] 421 4.7.0

Deferred: Volume, Complaints or Unverified

Observed with "temporarily deferred due to user complaints or because the IP/domain is in the unverified category". Reduce concurrent connections and sending rate, and register with the Yahoo Sender Hub.

🟡 [TSS04] 421 4.7.0

Deferred: Unexpected Volume or Complaints

Observed with "temporarily deferred due to unexpected volume or user complaints". Throttle your sending rate, retry with back-off, and check your complaint rate.

🟡 [TSS05] 421 4.7.0

Deferred: Unexpected Volume or Complaints

Carries the same sentence as TSS04 in reported bounces. Retry with back-off. Your mail server handles this automatically.

🔴 [TSS07] 550 5.7.0

Blocked: Spam or Policy

Reported as a permanent spam or policy block. We have found no Yahoo documentation for this identifier, so read the sentence beside it and check your reputation via the Yahoo Sender Hub.

🟡 [TS02] 421 4.7.0

Deferred (Legacy Format)

Older Yahoo bracket format, reported as connection throttling. Reduce concurrent SMTP connections and retry.

🔴 [TS06] 550 5.7.0

Blocked (Legacy Format)

Older format, reported as a reputation block. Undocumented by Yahoo. Review list hygiene, authentication and complaint rate before requesting review.

Yahoo Sender Hub Requirements

Yahoo (along with Google) updated its bulk sender requirements in 2024. To maintain good deliverability to Yahoo and AOL mailboxes:

Authenticate mail with SPF and DKIM aligned to your sending domain
Publish a DMARC policy (at minimum p=none)
Keep spam complaint rates below 0.3%
Implement one-click unsubscribe for marketing and subscription mail
Send only to users who have explicitly opted in

📖 Yahoo Sender Hub →

Frequently Asked Questions

Common questions about Yahoo and AOL SMTP errors.

What is the difference between [TSS] and [TS] codes?

[TSS] codes (TSS02, TSS04, TSS05, TSS07) are the current Yahoo / AOL identifiers; [TS] codes (TS02, TS06) are the older format. Yahoo publishes no index of either, so nobody outside Yahoo can tell you authoritatively what separates them. What is observable is that the temporary ones arrive with the same generic deferral sentence, and the fix is the same for all of them: slow down, cut concurrent connections, and get your complaint rate down. Treat the bracket as a reference number, not a diagnosis.

Why does Yahoo return 421 instead of 550 for spam blocks?

Yahoo is a bit sneaky about this: it often uses a 421 (temporary deferral) instead of a 550 (permanent block) as its first response to a suspicious sender. The idea is to make your mail server retry, which gives Yahoo a chance to watch your sending pattern over time. If you keep triggering their filters, those 421s can eventually harden into permanent blocks. Your mail server handles 421s automatically, so you may not notice them unless you check your logs.

How do I get my IP unblocked by Yahoo?

Head to senders.yahooinc.com, Yahoo's Sender Hub for exactly this situation. Registering for their feedback loop is the most important step: it shows you which of your messages Yahoo users are marking as spam, so you can fix the root cause rather than just waiting. There is also a complaint form on the same site for urgent unblock requests.

Does Yahoo use DMARC enforcement?

Yes, and Yahoo was actually one of the very first major providers to go all-in on DMARC. Yahoo's own domains (yahoo.com, aol.com) have had a p=reject DMARC policy for years, which is why you cannot spoof a Yahoo address. For mail arriving in Yahoo inboxes, Yahoo enforces the DMARC policy set by the sending domain. If your domain has p=reject and your message fails DMARC, Yahoo will reject it.