SMTP 421 4.7.0: too many connections, try again later, service not available
Why servers answer 421 (Gmail "Try again later, closing connection", "Too many concurrent SMTP connections", Office 365 "4.3.2 Service not active"): connection and rate limits, greylisting and overload. How to throttle, pool connections and retry correctly.
What 421 means: A temporary refusal of the whole connection: the server is telling you to come back later. 4xx codes are retryable — a correct client queues the message and retries with backoff.
Causes, most likely first — and the fix for each
- Too many simultaneous connections from your IP or account (Gmail 421-4.7.0 "Too many concurrent SMTP connections", cPanel/Exim per-host limits).
Fix: Limit concurrency to a few connections and reuse them (connection pooling); send many messages per connection instead of one connection per message. - Per-minute or per-hour rate exceeded (Office 365 30 messages/minute; Gmail bursts; hosting caps).
Fix: Throttle to the documented rate, or move volume to a service designed for it. - Greylisting on the receiving side ("421 4.7.1 Try again later" from rspamd/postgrey): the receiver wants you to retry to prove you are a real MTA.
Fix: Nothing — a proper mail server retries automatically within minutes. Scripts that give up on 4xx lose mail; use a queue. - Reputation-based deferral (Gmail 421-4.7.28 "unsolicited mail originating from your IP address"; Microsoft 421 RP-001/002).
Fix: Reduce volume, fix authentication, warm up the IP/domain; check Postmaster Tools / SNDS. - Server overloaded or restarting.
Fix: Retry with exponential backoff.
How to confirm which one you have
Count your open connections and your sends per minute against the provider's published limits; a relay with a queue and warm-up governor handles 421s for you.
Open the SMTP tester Check SPF / DKIM / DMARC
Still stuck? Use a relay that tells you why
QueenSMTP shows every message's exact server reply in the log, verifies your domain's SPF and DKIM before you send, and is not subject to mailbox-provider caps. $5/year plus $0.10 per 1,000 emails; 100 a day free to test.
Try it freeRelated errors
Questions about 421
Yes — 4xx means try again. Retry after 1–5 minutes with exponential backoff and a cap; do not hammer the server every second.
421 drops the connection; 450/451 refuse one command (recipient or message) but keep the session open. All three are temporary.
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