Yahoo 421 4.7.0 [TSS04] Temporarily Deferred

What’s actually happening

Mail to yahoo.com, aol.com, and other Yahoo-hosted domains stops going through and piles up in your queue. Your MTA keeps retrying and keeps getting the same 421 line back. Nothing actually bounces yet — it's a 4xx, so the messages sit in deferral and only hard-bounce once they age past your queue lifetime (often 3-5 days). Other providers like Gmail and Outlook usually keep accepting fine, which is the tell that this is Yahoo-specific reputation, not your server.

Common causes

• A cold or recently-rotated IP sending real volume before it was warmed up — Yahoo has no history for it and defers by default.
• A volume spike: you 5x'd your daily send to Yahoo addresses overnight and tripped their rate model.
• Complaint rate climbing above roughly 0.3% — Yahoo users hitting 'Report Spam' is the fastest way into TSS04.
• A shared ESP pool where another tenant on the same IP poisoned the reputation.
• Stale or purchased lists generating bounces and spam-traps hits, which Yahoo reads as low-quality sending.

How to fix it

1. Let it ride first — this is a temporary deferDon't panic-change DNS or move IPs. A 421 means 'come back later.' Confirm your MTA is honoring retry/backoff and not hammering Yahoo every 30 seconds, which only reinforces the bad signal. Give it ~4 hours before expecting recovery.
2. Cut your Yahoo send rate immediatelyThrottle outbound to Yahoo domains down to a fraction of current volume — in Postfix, set a per-destination concurrency/rate limit for yahoo.com et al. (smtp_destination_rate_delay, smtp_destination_concurrency_limit) so you trickle instead of flood while reputation recovers.
3. Enroll the sending domain in Yahoo's Postmaster toolsSign up at postmaster.yahooinc.com, verify the domain, and check your complaint rate and spam-trap signals. Also enroll in their Complaint Feedback Loop (CFL) so you get told which recipients complained and can suppress them.
4. Verify SPF, DKIM, and DMARC all pass for the sending domainYahoo enforces the bulk-sender rules: aligned SPF or DKIM plus a DMARC policy, plus one-click List-Unsubscribe (RFC 8058) on marketing mail. Missing any of these makes deferrals stickier and turns them into outright blocks over time.
5. Scrub the list and warm the IP back upRemove hard bounces, role accounts, and anyone who hasn't engaged in months. Then ramp Yahoo volume gradually over days rather than jumping back to full blast — a fresh spike right after recovery just lands you back in TSS04.