421 Service not available, closing transmission channel
Temporary: it may work if you retryThe receiving server is not able to talk right now and is closing the connection. It is a temporary failure.
Usual causes
- The server is overloaded or being restarted.
- It is limiting how many connections or messages it accepts from you (often a rate limit or a reputation throttle).
- It may have temporarily blocked your sending address because of a spike in volume.
What to do
- Retry later. Good mail systems retry for you, backing off between attempts.
- If it keeps happening, slow down: send fewer messages at a time to that provider, and spread large sends over hours.
- Read the text after the code. Providers often say why, and link to a page about their limits.
Usually the fault of: receiving server.