421 Service Temporarily UnavailableWhat does 421 Service Temporarily Unavailable mean?
The 421 code means the receiving mail server is temporarily unavailable and cannot process your message right now. This is a transient failure: the server is telling your sending MTA to try again later. It does not indicate a problem with the recipient address or your sender reputation in most cases. Common triggers include server overload, maintenance windows, connection limits, and brief network outages on the receiving side.
Is this a soft or hard bounce?
Soft bounce. Do not remove the address. A 421 is a temporary deferral. Your sending server will automatically retry delivery according to its retry schedule, typically every 15 to 30 minutes for the first few hours, then less frequently over the next 24 to 72 hours. Only escalate to a hard bounce if the 421 persists beyond your MTA retry window without resolving.
Common causes
Server overload: The receiving server has too many simultaneous connections and is refusing new ones temporarily. This is common with large providers during peak sending hours.
Rate limiting by the receiving server: The server is throttling your IP or domain because you are sending too fast. Large-scale senders hitting Gmail or Outlook often see 421s when they exceed connection thresholds. This relates directly to inbox placement and sender reputation.
Maintenance or restart: The remote server is undergoing scheduled maintenance or restarting its SMTP service. 421s during a narrow time window often resolve on their own within minutes.
Greylisting: Some servers use greylisting as a spam filter. They issue a 421 on the first delivery attempt from an unknown sender and accept the message on the retry. See the full SMTP error codes reference for how greylisting interacts with other 4xx codes.
How to fix it
Step 1: Wait and let your MTA retry. Most 421 errors resolve automatically within the standard retry window. Check your mail queue before taking any manual action.
Step 2: Check your sending volume and throttle if needed. If you are sending a large campaign, reduce your connection rate or spread the send over a longer window. Most ESPs let you configure sending speed.
Step 3: Check your sender reputation. Persistent 421s from major providers like Gmail can indicate that your IP is being throttled due to reputation issues. Check your IP against major blocklists before retrying at scale.
Step 4: Review your authentication setup. Ensure SPF, DKIM, and DMARC are correctly configured. Authentication failures can cause receiving servers to issue 421s before escalating to permanent rejections.
Provider-specific notes
| Provider | Behavior |
|---|---|
| Gmail | Returns 421 4.7.0 when your IP is sending too fast or has triggered volume thresholds. Automatic retry usually succeeds within 1 to 4 hours. |
| Outlook / Exchange | Returns 421 4.3.2 Service currently unavailable during maintenance or when the server is under load. Retry after 30 minutes. |
| Postfix | Logs a 421 when the remote server drops the connection. Check /var/log/mail.log for the full SMTP conversation. |
| Yahoo | Returns 421 Service temporarily unavailable when your sending IP exceeds hourly limits. Slow down your send rate. |
Related error codes
421 4.7.0 Too many connections · 421 4.2.1 Please try again later · 451 Greylisted · 421 4.7.0 Messages temporarily deferred
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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