// how to fix
Yahoo and AOL "421 4.7.0 [TSS04]" deferrals
Yahoo is temporarily refusing your emails because of their volume or complaints. Slow down, keep retrying, and fix what Yahoo does not like.
What it means
When you send to Yahoo, AOL or other addresses handled by Yahoo, your server receives an answer like:
421 4.7.0 [TSS04] Messages from 203.0.113.10 temporarily deferred due to unexpected volume or user complaints
Codes starting with 4 are temporary: Yahoo is not rejecting the email for good, it is asking your server to try again later. TSS04 means Yahoo is unhappy with the reputation of your IP or domain, usually because of a sudden jump in volume or because recipients marked your emails as spam.
Why it matters
- Emails arrive late, sometimes hours later.
- If the situation lasts, your server eventually gives up and the email bounces.
- It is an early warning: ignoring it can lead to permanent blocks.
How to check
- Read your server logs or your sending tool's bounce reports for
TSS04. - Send a test to your address on the home page (
/). Yahoo requires SPF or DKIM for everyone, and both plus DMARC for bulk senders. Check that the report shows them passing and aligned with your From domain. - Check your IP and domain at
/blacklists.
How to fix it
1. Let your server retry
Do not resend manually and do not delete the queue. A normal mail server retries deferred emails automatically for several days. Sending again makes the volume problem worse.
2. Reduce your sending speed to Yahoo
- Send campaigns in smaller batches spread over hours.
- On your own server, limit the number of simultaneous connections and messages per connection to Yahoo domains.
- If you changed IP or provider recently, ramp up slowly over a few weeks.
3. Lower complaints
- Send only to people who asked for your emails.
- Make unsubscribing easy, with a visible link and the one-click unsubscribe header.
- Sign up for Yahoo's Complaint Feedback Loop to see who complains, and remove them.
- Remove subscribers who have not opened anything for months.
4. Clean your list
Many old or invalid Yahoo addresses in one campaign hurt your reputation. Remove addresses that bounced and verify old lists before using them again.
5. Get authentication right
SPF, DKIM with your own domain and a DMARC record. A valid reverse DNS on your sending IP. These are basics Yahoo expects.
Yahoo explains its rules and error codes on the Yahoo Sender Hub.
Check that it is fixed
TSS04 deferrals usually fade within hours to a few days once volume and complaints drop. Watch your logs: fewer and fewer 421 answers from Yahoo. If they continue after a week of clean, slow sending, contact Yahoo through the Sender Hub with your IP and examples.
Check that it is fixed
DNS changes can take from a few minutes to a few hours. Then send a new test email or scan your domain again.
Related guides
- Your emails land in Gmail's spam folderWhen Gmail sends your emails to spam, the cause is almost always authentication, reputation or content. Here is how to find which one and fix it.
- Your emails land in Outlook or Hotmail junkOutlook.com, Hotmail and Live send many new senders to Junk. Check your setup, then use Microsoft's free sender tools: SNDS, JMRP and the delist portal.
- Gmail bounce "550 5.7.26": unauthenticated emailGmail refused your email because it could not confirm it comes from your domain. SPF, DKIM or DMARC is missing or failing. Here is how to fix it.
- Bounce "550 5.7.1": message rejected or blocked by policyA 5.7.1 bounce means the receiving server refused your email on purpose. Learn to read bounce messages, find the real reason and fix it.
- Warm up a new domain or IP addressA new domain or IP has no reputation yet. Sending a large campaign on day one looks like spam. Start small and increase your volume step by step.