If Gmail is dropping your connections with "421 4.7.0", it is rate-limiting your IP. You are opening too many SMTP connections from one address in a short window. The good news: this is a temporary failure, not a permanent block. Slow down, and the errors stop. Here is how to do that without losing volume.
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Run the free checkThe first digit, 4, means a temporary failure: the connection was dropped and the server is asking you to try again later. The enhanced code 4.7.0 is a delivery rate limit. Gmail is not blocking your IP permanently; it is telling you that your address is opening too many SMTP connections at once and to back off.
1. Too many concurrent connections. Opening dozens of SMTP sessions from one IP at the same time trips Gmail's connection rate limit.
2. Sending too fast. A burst of high volume in a short window looks like an attack, even when the mail is completely legitimate.
3. A cold IP sending high volume. A brand new IP with no sending history that suddenly sends thousands of messages gets rate limited.
1. Cap your concurrent connections. Keep the number of simultaneous SMTP sessions per IP low and let your mailer queue the rest instead of opening everything at once.
2. Warm up the IP. Start with low volume and increase gradually over days or weeks so the reputation builds before you push hard.
3. Spread volume across IPs. Use multiple sending IPs or domains so no single address hits the rate limit on its own.
4. Use an ESP that manages rate limits. Reputable ESPs throttle sending automatically and rotate IPs for you, which removes the problem entirely.
Related: SMTP error codes explained · Gmail SMTP error 550-5.7.26 · Cold email deliverability · Why email lands in spam