SMTP Reply Codes
SMTP 421 — Service not available, closing transmission channel
In SMTP Reply Codes, code 421. The official source describes it as “Service not available, closing transmission channel”. Its concrete cause depends on the operation and surrounding message.
Code421
Symbol—
Source confidencehigh
Official definition
Service not available, closing transmission channel
In plain language
In SMTP Reply Codes, code 421. The official source describes it as “Service not available, closing transmission channel”. Its concrete cause depends on the operation and surrounding message.
Context to inspect
- The surrounding operation, version and subsystem determine the actual cause.
- The same numeric value can mean something different in another namespace.
Safe first checks
- Confirm which product, protocol or subsystem produced the code.
- Read the full surrounding error message and inspect relevant logs.
- Compare the code with the linked authoritative documentation before changing configuration.
Important context
Exact wording and enhanced status detail can vary by server; 4xx is generally transient and 5xx permanent. Bare SMTP reply codes are context-dependent; enhanced status codes may provide more detail.