Is 421 4.7.0 [TSS04] permanent or temporary?

421 4.7.0 [TSS04] is a temporary (4.x.x) failure — a properly configured mail server retries automatically, and the message often delivers on a later attempt. Persistent 421 4.7.0 [TSS04] across recipients points to a deeper issue.

Is this my problem or the recipient's?

Often it resolves itself on retry. If it persists across many recipients, it's usually a sending-side reputation or authentication issue worth fixing.

How do I check my sending setup?

Run a free EmailHealth check on your domain — it verifies SPF, DKIM, DMARC, blacklist status, and more in a couple of seconds, which covers the most common causes of delivery failures.

421 4.7.0 [TSS04] — Yahoo temporarily deferred your mail

421 4.7.0 [TSS04] is a temporary SMTP reply meaning yahoo temporarily deferred your mail. Yahoo groups its temporary deferrals as "TS" errors. Its sender documentation names the causes: many complaints from Yahoo users, content Yahoo objects to, a poor IP reputation, or unusual traffic. Yahoo says to review your outgoing mail and queues, and to contact Yahoo Sender Support if the error does not stop.

What 421 4.7.0 [TSS04] means

Yahoo groups its temporary deferrals as "TS" errors. Its sender documentation names the causes: many complaints from Yahoo users, content Yahoo objects to, a poor IP reputation, or unusual traffic. Yahoo says to review your outgoing mail and queues, and to contact Yahoo Sender Support if the error does not stop.

ℹ️

This is a 4.x.x temporary failure — a correctly configured mail server retries on a backoff schedule, and the message frequently delivers on a later attempt.

Common causes

📨

Yahoo users marked your mail as spam

Most common

High complaint rates are the most common reason. Yahoo, like Gmail, expects bulk senders to keep complaints low and to offer one-click unsubscribe.

🔁

A sudden rise in volume or unusual traffic

Common

A new IP or domain, a cold domain, or a large send after a quiet period looks suspicious.

⚙️

Poor IP reputation or authentication problems

Common

A shared IP with bad neighbours, or SPF, DKIM or DMARC that does not pass for bulk mail.

How to fix 421 4.7.0 [TSS04]

1
Let your server retry slowly
Do not resend by hand. Lower the sending rate to Yahoo and let the queue retry over several hours.
2
Check authentication and blocklists
Bulk senders to Yahoo need SPF, DKIM and DMARC. Run a free EmailHealth check to confirm they pass and that the domain is not listed.
3
Reduce complaints, then ask Yahoo
Send only to people who opted in, remove inactive addresses, add one-click unsubscribe, and join the Yahoo Complaint Feedback Loop. If deferrals continue, submit a Sender Support Request at senders.yahooinc.com.

Confirm your domain is set up correctly

Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.

Yahoo temporarily deferred your mail