421 Connection refused, too many connections from your IPWhat does 421 Too many connections mean?
The receiving server enforces a maximum number of simultaneous SMTP connections per source IP, and your sending pattern exceeded that limit. Common limits range from 5 to 50 concurrent connections depending on the receiver. The 421 response is soft: retries will succeed once your concurrent connection count drops. The fix is sender-side: reduce the maximum concurrency in your MTA configuration so you stay within the receiver’s limits, particularly for high-volume domains like Gmail, Outlook, and Yahoo which enforce strict per-IP caps.
Is this a soft or hard bounce?
The fix is on your side: throttle outbound concurrency. Most queue managers can be configured per destination domain to respect these limits without manual intervention.
Common causes
Postfix default smtp_destination_concurrency_limit = 20 can be too high for some receivers.
ESPs or custom mail systems blasting to a single domain without grouping or per-domain limits.
Application bug holding open connections beyond message delivery. Audit code for proper QUIT and connection close.
How to fix it
Bounce text typically includes the recipient domain. Major receivers (Gmail, Outlook, Yahoo) have stricter limits than smaller MX hosts.
Postfix: smtp_destination_concurrency_limit = 5 for problematic destinations. Exim: connection_max_messages and remote_max_parallel.
Send to recipients on the same domain in batches, not interleaved with others. Reduces connection churn.
Confirm every SMTP session ends with QUIT before connection close. Long-lived connections without QUIT exhaust per-IP slots.
Provider-specific notes
| Major receiver | Per-IP connection limit (approximate) |
|---|---|
| Gmail | Hard cap around 10 simultaneous connections per IP. |
| Outlook / Office 365 | Around 20 simultaneous connections per IP, but degrades faster on poor reputation. |
| Yahoo / AOL | Around 20 simultaneous; very strict on velocity and pattern. |
Related error codes
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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