What the message looks like
421-4.7.0 Try again later, closing connection421-4.7.28 Gmail has detected an unusual rate of unsolicited mail421 4.7.0 [TSS04] Messages temporarily deferred due to unexpected volume or user complaints421 4.7.0 TLS required
What it means
The receiving server temporarily refused the email for a security or policy reason and closed the connection. The standard name for this status is "other or undefined security status". Because it starts with 4, the sending server keeps the email and retries.
Common causes
- The sending IP address is sending a lot of email, or a lot of email that people mark as spam, so the receiving server is slowing it down. Gmail uses
421 4.7.28for this, and Yahoo adds codes like[TSS04]or[TS02]. - The receiving server requires an encrypted (TLS) connection and the sender didn't use one.
- The sending server's IP address has a poor or unknown reputation, such as a new server.
- A temporary problem at the receiving end.
If you're the recipient
Usually nothing. Most emails delayed like this arrive within a few hours. If they keep failing, let the sender know.
If you're the sender
Let your server keep retrying. If it happens often, check your sending volume, complaint rate and authentication, and make sure your server supports TLS. Gmail's Postmaster Tools can show your domain's reputation with Gmail. Gmail also sends temporary 421 4.7.26, 4.7.27, 4.7.30, 4.7.32 and 4.7.40 errors when SPF, DKIM or DMARC isn't set up properly, as a warning before it starts rejecting. See 550 5.7.26.
If you use addy.io
If your inbox provider sends this error when addy.io forwards to you, addy.io retries automatically, so the email is usually just delayed. See Why are forwarded emails delayed?