Transient (4xx)
421 4.3.2
Service not available, closing transmission channel
The receiving server is temporarily too busy or is deliberately dropping the connection.
421 4.3.2 Service not available, closing transmission channel
What it means in practice
421 is the one reply that ends the whole SMTP session, not just the current message. The receiving server closes the connection, so every message queued on that connection gets deferred, not only the one in flight.
The usual triggers are planned restarts and overload protection. Some providers also use 421 when a single sending IP opens too many parallel connections. If the deferrals line up with your own traffic peaks, lower the concurrency per destination domain before you look anywhere else.
Common causes
- Receiving server is overloaded and shedding load.
- Sending IP has hit a temporary per-connection or per-hour limit at that provider.
What to do
- →Retry with backoff - this is transient by definition (4xx) and should clear on its own.
- →If it happens consistently to one provider, check your sending rate and IP reputation with that provider specifically.