Yahoo Mail and AOL — both operated by Yahoo Inc. — embed proprietary deferral codes in square brackets inside standard SMTP responses. You will see codes like [TSS02] or [TS06] mixed in with the usual SMTP text, and they carry a specific meaning. This directory covers both the standard codes with Yahoo-specific behaviour and the proprietary [TSS]/[TS] codes — with a plain-English explanation and a fix for each.
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] code identifies the specific Yahoo throttling reason. ec=ax9 [16] is an internal transaction identifier and can be ignored.
These standard SMTP codes have documented Yahoo-specific behaviour or responses.
These bracketed codes appear inside Yahoo's SMTP rejection text. They provide more specific reasons than the outer SMTP code alone.
Yahoo (along with Google) updated its bulk sender requirements in 2024. To maintain good deliverability to Yahoo and AOL mailboxes:
p=none)Common questions about Yahoo and AOL SMTP errors.
[TSS] codes — TSS02, TSS04, TSS05, TSS07 — are the current Yahoo / AOL identifiers, used since Yahoo consolidated its mail infrastructure. [TS] codes — TS02, TS06 — are the older format. Both mean the same things; which one you see depends on which Yahoo mail cluster handled your connection. When in doubt, treat them identically.
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.
Head to senders.yahooinc.com — this is 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.
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 — so if your domain has p=reject and your message fails DMARC, Yahoo will reject it.
Paste your full NDR email or SMTP error line for an instant plain-English diagnosis.
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