smtp;421 4.3.2 Service not available [outlook.com]
The receiver deferred the message and points at a mail-system problem on the receiving side (enhanced status 4.3.2). Use the wording above to pin down the exact fix.
Email provider · 4.x.x transient failure
Outlook SMTP 421 Errors: Every Bounce Response Explained
Every SMTP 421 response on record from Outlook (hotmail.com, outlook.com, msn.com, live.com), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender.
- Provider
- Outlook
- Code
- 421 (4xx)
- Responses
- 1
- Highest severity
- Low
Outlook SMTP 421 responses
Outlook SMTP 421 questions
›What does an SMTP 421 error from Outlook mean?
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender. At Outlook, it is a temporary deferral that is retried automatically, and the specific reason is in the enhanced status code and message text of each response below.
›How do I stop Outlook from returning SMTP 421?
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Outlook's postmaster documentation.