smtp bounce code
421 4.7.0Temporary rejection, try later
what to do
Retry, do not suppress
A 4xx code is temporary by definition. Nothing here says the address is bad, and suppressing on it throws away good contacts.
What the server is telling you
The server is rate limiting, greylisting, or declining to talk right now. Greylisting in particular refuses the first attempt from any unknown sender on purpose, and accepts the retry.
Next steps
- →Retry. A normal mail server does this automatically.
- →If you are probing rather than sending, this is why single-attempt verification is unreliable.
- →Repeated 421s from one destination usually mean you are sending too fast.
Commonly returned by: Everything, especially under load
Find these before you send, not after
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