421 4.7.29: not sent over TLS, rate limited
Gmail is rate limiting your mail because your server delivered it without TLS encryption, which Google's sender guidelines require.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 4.7.29
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
421 4.7.29 Your email has been rate limited because you're not using a TLS connection.
What it means
Google's sender guidelines list a TLS connection among the requirements for mail sent to Gmail; the reply itself speaks of bulk senders. Your server delivered without starting TLS, so Gmail has rate limited the message. This is the temporary form, also listed in Google's sender guidelines FAQ; the permanent form is 550 5.7.29. Gmail's servers offer STARTTLS, so the cause is almost always on the sending side: STARTTLS switched off, a failed handshake that the server retries in plain text, or a device in the path that hides the STARTTLS offer.
Common causes
- STARTTLS is turned off for outgoing mail on the sending server.
- TLS software too old to complete a handshake with Gmail, followed by a plain-text retry.
- A firewall or security appliance that removes STARTTLS from the SMTP conversation.
How to fix it
- 01Turn on STARTTLS for outgoing mail on your server, or ask your provider to.
- 02Update the server's TLS libraries to support current protocol versions.
- 03Remove any SMTP inspection that strips STARTTLS, then send a test to a Gmail address.
- 04In the test message, check the Received header for an encrypted (ESMTPS) hop.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
- Email sender guidelines FAQ (Gmail Help)support.google.com
- Email sender guidelines (Gmail Help)support.google.com
Related codes
- 550 5.7.29Not sent over TLS, blocked.Gmail rejected the message because your server delivered it without TLS encryption, which Google's sender guidelines require.
- 421 4.7.0TLS required by the recipient's domain.The Workspace domain you are writing to only accepts mail over an encrypted connection, and your server tried to deliver without TLS.
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
Further reading
All 108 Gmail codesEvery code in the wikiPaste your own bounce