← Glossary
Delivery
What is Throttling (deferral)?
A receiving server temporarily slowing or refusing mail from a sender, usually with a 421 or 450 reply.
How it works in practice
Providers throttle new senders and anyone whose volume or reputation looks risky. The mail isn't lost: the sending server retries. If throttling persists the right response is to reduce volume and fix whatever hurts reputation.
The common mistake
Increasing parallel connections to push mail through a throttle.
Check it yourself
Related terms
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