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.
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.