Yahoo "421 4.7.0 [TSS09] All messages from [IP] will be permanently deferred; Retrying will NOT succeed": what to do

What it means

Yahoo Mail (and AOL Mail) has decided not to accept any email from your sending IP address for now. The message usually looks like this:

421 4.7.0 [TSS09] All messages from 203.0.113.10 will be permanently deferred; Retrying will NOT succeed. See https://...

A 421 code is normally a temporary error: the sending server keeps the email and tries again later. Here Yahoo warns that retrying will not help. Your server will keep trying for a while and then give up with a bounce.

It is different from the milder [TSS04] code, which means "temporarily deferred due to user complaints". TSS09 is the stronger version: Yahoo's systems rate your IP as a bad source of mail.

Why it happens

Yahoo does not give a precise reason, but TSS09 is about reputation. Common causes:

• Many spam complaints. Yahoo users clicked "Spam" on your messages.
• Old or bought lists with many addresses that no longer exist, or spam traps (addresses used only to catch bad senders).
• A sudden volume spike from an IP that usually sends little, or a brand new IP with no history.
• A hacked account or form on your server sending spam without you knowing.
• Missing authentication: no SPF, no DKIM or no DMARC, which makes your mail harder to trust.
• A shared IP where another customer of your provider damaged the reputation.

How to check

1. Run a Delivmail test. From the same server or tool, send an email to the unique address shown on the home page (/). Read the report: SPF, DKIM and DMARC must pass, reverse DNS must be correct, and the IP should not be on blocklists.
2. Check blocklists. Enter the IP at /blacklists. A listing on Spamhaus or others is a strong hint that something is sending spam.
3. Read your logs. Look at how many emails went to Yahoo and AOL addresses in the last days. An unexpected peak often means a compromised account.
4. Use Yahoo Sender Hub. At senders.yahooinc.com you can read Yahoo's sender requirements, sign up for the Complaint Feedback Loop (Yahoo sends you a copy of each complaint) and find the form to contact Yahoo's postmaster team.

How to fix it

1. Stop the source. If you find spam you did not send, change passwords, fix the form or script, and clean the mail queue.
2. Pause sending to Yahoo and AOL for bulk campaigns while you fix the rest. Keep only important one-to-one emails.
3. Clean your list. Remove addresses that bounced, people who never open, and anyone who did not clearly agree to receive your emails. Never buy lists.
4. Join the Complaint Feedback Loop and unsubscribe every person who complains, automatically.
5. Add a clear unsubscribe link and the one-click unsubscribe header for marketing mail.
6. Authenticate everything: SPF, DKIM with your own domain, and a DMARC record.
7. Contact Yahoo through the Sender Hub form once the cause is fixed. Explain what went wrong and what you changed. Be honest and short.
8. On a shared provider, send the bounce to your provider's support: they manage the IP.

Restart slowly

When mail starts flowing again, increase volume gradually over days, starting with your most engaged recipients. A new burst can bring the block back.

Check that it is fixed

Send a new Delivmail test and confirm authentication passes and the IP is clean on blocklists. Then send a single email to a Yahoo address you control. Reputation recovery is not instant: it can take from a few days to a couple of weeks of clean, low-complaint sending. Keep an eye on complaints from the feedback loop during that time.