How long does a Yahoo TSS04 deferral last?

There is no fixed duration. Yahoo re-evaluates on each connection; many senders see acceptance resume within a few hours of reducing their rate, and a new IP can be deferred on and off for a couple of weeks while its reputation forms.

Should I open a ticket with Yahoo for TSS04?

Not for a plain TSS04. Yahoo's sender support treats it as normal rate control and asks you to follow the best practices page. Open a ticket only if deferrals continue for days at low volume with no complaints.

Why does the deferral mention "user complaints" when I only send transactional mail?

The wording is the same for every TSS04 and does not mean complaints were the trigger. Volume alone, or an IP with no history, produces the identical message.

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

Yahoo is slowing your IP down. TSS04 is its standard deferral when an IP sends more than Yahoo expected or when Yahoo users have been hitting "spam" on its mail. The messages are not lost; your server will retry them, and Yahoo accepts them as the rate comes down. Because AOL mail runs on Yahoo's infrastructure, the same code arrives from aol.com addresses.

What the server replies

421 4.7.0 [TSS04] Messages from 203.0.113.10 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

Sent by: Yahoo and AOL
Type: TemporaryRetry helps
About: Rate limits and quotas
Enhanced code: 4.7.0

Who sends it

Yahoo and AOL inbound servers (mta5.am0.yahoodns.net and friends), at RCPT TO or DATA, keyed to the sending IP. It is by far the most common Yahoo deferral and shows up for almost every new sender.

Why it happens

Volume to Yahoo jumped compared with the IP's recent history.
Complaints from Yahoo users in the last day or two, which Yahoo weights heavily and quickly.
A new IP with no history at all; Yahoo starts everyone at a low allowance.
Sending to many addresses that are inactive, which Yahoo treats like spam traps.

How to fix it

Retry slowly and keep sending
Do not stop entirely; Yahoo builds its allowance from accepted mail. Set a low connection count and a per-message delay toward yahoo.com, aol.com and the other Yahoo-hosted domains, and let the queue drain over hours.
Join the Complaint Feedback Loop
Yahoo's CFL sends you every complaint for a DKIM domain. Remove complainers immediately; a complaint rate above 0.1 percent at Yahoo brings TSS04 back within hours.
Prune inactive addresses
Yahoo recycles long-dead accounts as traps. Stop sending to anyone who has not opened or clicked in the last six to twelve months.
Warm up a new IP over three weeks
Start at a few hundred Yahoo recipients a day, your most engaged ones, and increase by roughly half every few days.

Check it yourself

These free tools show the records and connections behind this code for your own domain or server.

Related codes

Sources

Fewer bounces to look up.

Faivelo queues temporary failures and retries them on a schedule receivers accept, then tells your application once, in plain words, if a message finally cannot be delivered.

Send with Faivelo

Free tier included. No card needed.