Sending or connection rate limited
Sending volume or concurrent connections have reached a limit.
What to do now
- Wait for the interval requested by the server, then check the sending result.
- If you operate the mail server, inspect and reduce the sending rate and concurrent connections.
Representative reply
421 Too many concurrent SMTP connections
Retry decision
Reduce sending rate and observe the requested waiting period.
Why it happens
Concurrent connections or sending volume may exceed a receiver limit.
Typical stage
CONNECT
- CONNECT
- EHLO
- MAIL FROM
- RCPT TO
- DATA
- DELIVERY
Stages vary by server. Confirm the actual stage from the complete reply and session logs.
What is SMTP Reply Code?Open only the explanations you need
Three-digit server reply: 4xx is transient, 5xx permanent. Read the complete text to investigate the cause.
What is Enhanced Status Code?Open only the explanations you need
An extended status such as 5.1.1. Server wording and usage may vary.
Check directly
- Wait for the interval requested by the server, then check the sending result.
- If you operate the mail server, inspect and reduce the sending rate and concurrent connections.
If you operate or diagnose the mail server, the tools below can help with these checks.
Cautions
Observed DSNs do not map one-to-one to causes. The stage shown is typical; confirm it with the full reply and actual logs.
Similar reply wording and observed status codes
too many concurrent smtptoo many connectionyou sent too many messagesmail too quicklyrate limited
Observed DSNs: 4.2.1, 4.7.26, 4.7.28