Yahoo 421 4.7.0 [TSS04] 'Temporarily Deferred': What It Means and How to Fix It

421 4.7.0 [TSS04] Messages from 203.0.113.25 temporarily deferred due to unexpected volume or user complaints means Yahoo's mail servers are slowing down how much mail they accept from your sending IP. It is a temporary failure: your server holds the message and retries. Yahoo's Sender Hub lists the usual causes as user complaints, content that looks like spam, poor IP reputation and unusual traffic. The fix is to send less, send to people more likely to want it, and meet Yahoo's sender requirements. Pushing more volume through makes it last longer.

Reading the deferral

421 4.7.0 [TSS04] Messages from 203.0.113.25 temporarily deferred due to unexpected volume or user complaints

• 421 is a temporary failure. The standard says the sender should try again later.
• 4.7.0 is the enhanced code for a security or policy reason, with no more detail.
• [TSS04] is Yahoo's own tag. Yahoo groups these under "Message temporarily deferred (eg: TS* Errors)".
• The IP is the one Yahoo is judging: your server's public IP or your provider's shared IP.

Nothing in the line is about one recipient. Every Yahoo address you mail from that IP is affected, and so are AOL addresses, which run on the same infrastructure.

What Yahoo says

Yahoo's SMTP error codes page (read 2 November 2026) says of TS errors: "This is a temporary error and your mail server may automatically re-try sending the email at a later time." It lists possible causes including substantial complaints from Yahoo users, content that looks like spam, a poor IP reputation, and unusual traffic from your mail servers.

For errors that continue, it suggests submitting a Sender Support Request with the specific error and the diagnostic codes from your logs. If you do not run the mail server yourself, it recommends contacting whoever does.

Yahoo does not publish how long TSS04 lasts or exactly what triggers it. Anyone who gives you a precise number is guessing.

Why retrying harder makes it worse

A deferral is Yahoo telling you to slow down. If your server, or a cold email tool that adds new sends on top of the retries, keeps pushing at the same rate, Yahoo sees the same behaviour that triggered the deferral and keeps deferring. Messages sit in the queue, retries pile up, and eventually they expire as delayed bounces. Bounces then make the next week's sending look worse.

How to fix TSS04

1. Slow down to Yahoo and AOL

Reduce the volume going to Yahoo-hosted domains (yahoo.com, aol.com and the others listed in Yahoo email domains) for a few days. If your platform cannot throttle by provider, send Yahoo and AOL leads from a separate campaign at a lower daily cap. Splitting by provider is easy with the bulk email provider lookup.

2. Look at complaints

Yahoo's best practices page (read 2 November 2026) says to "keep your spam rate below 0.3%". It asks bulk senders to "enroll in the Complaint Feedback Loop (CFL)" and says "an active CFL is needed for all DKIM domains". The CFL reports Yahoo users who marked your mail as spam, so you can remove them. Work out your current rate with the spam complaint rate calculator.

For cold email, complaints come from sending to people who had no reason to expect your mail. Tighter targeting and fewer follow-ups to non-responders lower them faster than any technical fix.

3. Meet Yahoo's sender requirements

From the same best practices page:

Requirement	All senders	Bulk senders
Authentication	SPF or DKIM at minimum	Both SPF and DKIM
DMARC	Not listed	A valid policy of at least p=none, and DMARC must pass
Spam rate	Below 0.3%	Below 0.3%
Unsubscribe		One-click list-unsubscribe header for marketing and subscribed mail, honoured within 2 days

Check your domain against both Gmail's and Yahoo's rules at once with the bulk sender checker. Our Gmail and Yahoo bulk sender requirements post compares the two in detail, and one-click unsubscribe explains the header.

4. Remove addresses that do not belong on the list

Yahoo also asks senders to remove invalid recipients promptly. Mailing abandoned Yahoo accounts is a reputation signal in its own right. Yahoo's help pages say a mailbox not used for 12 months becomes inactive, so old Yahoo addresses on a list are a real risk.

There is a catch. Yahoo's documented error for a missing user arrives as a reply to the message, not during the recipient check, so no verifier can confirm a Yahoo mailbox in advance. Our why Yahoo addresses are hard to verify post explains the detail. What a verifier can do is remove bad syntax, typo domains and dead domains. For the rest:

• Keep old Yahoo addresses (from a list more than a year old, or never engaged) out of cold campaigns.
• Send to recent, engaged Yahoo addresses first while you recover.
• Suppress every Yahoo hard bounce immediately so later steps do not mail it again.

5. Ramp back up gradually

Once deferrals ease, increase Yahoo volume in steps over several days rather than jumping back to your old rate. A sudden jump is one of the "unusual traffic" patterns Yahoo names.

6. File a Sender Support Request if it persists

If deferrals continue after a week or more of lower volume, clean authentication and low complaints, file a Sender Support Request through Yahoo's Sender Hub. Include the full deferral line, timestamps, the sending IPs and what you changed.

TSS04 vs the other Yahoo codes

Code	Type	What it means	Page
421 4.7.0 [TSS04]	Temporary	Deferred for complaints, volume or reputation	Yahoo TSS04
[TSS09]	Reported as lasting	All mail from the IP deferred; retrying does not help	Yahoo TSS09
554 delivery error ... doesn't have a yahoo.com account	Permanent	The mailbox does not exist	Yahoo 554 delivery error

The 554 is a list problem. TSS04 and TSS09 are sender problems. If you are not sure which side your bounces are on, read is it my list or my domain?.

Preventing the next one

• Ramp new IPs, domains and inboxes slowly, especially to Yahoo and AOL.
• Keep a steady daily volume instead of large bursts.
• Stay below 0.3% complaints and process CFL reports.
• Keep unengaged Yahoo addresses out of cold campaigns and suppress bounces after every step.

Where SimpleVerifier fits

SimpleVerifier cannot confirm Yahoo or AOL mailboxes, and says so: they come back risky, never valid, after syntax, domain, typo, disposable and role checks pass. What it does do is remove the addresses that are clearly bad and confirm mailboxes at providers that answer, so the rest of your list does not add to the problem. The free Yahoo email checker shows what can and cannot be checked for one address.

The first 100 checks are free with a Google sign-in on the verify page. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.