Glossary
Throttling
Also called rate limiting, deferral.
Last reviewed 18 September 2026
What is throttling in email sending?
Throttling is a receiving provider deliberately slowing or temporarily refusing mail from a sender, usually with a 4xx response, because the volume, the pattern or the reputation it is seeing is more than it is willing to accept right now.
From the sending side it looks like a rise in soft bounces and deferrals at one provider and nowhere else, which is the tell. The correct response is to send more slowly and find out what changed, not to retry harder: a sender that answers a deferral with more connections is confirming the reason it was deferred.
A course fans out one message per subscriber per day rather than one blast to everybody, which spreads volume naturally and is one of the quieter advantages of the format.
See also
- Soft bounce
A soft bounce is a temporary refusal of a message by the receiving server, carried in a 4xx SMTP response, because the mailbox is full, the server is greylisting you, or you are being rate-limited.
- Domain warming
Domain warming is the practice of raising the volume sent from a new domain gradually, over something like two to four weeks, so that a domain with no reputation does not arrive looking like one registered this morning for a campaign.
- Idempotency key
An idempotency key is a value a sender attaches to a request so that a provider receiving the same request twice performs the work once, which is what makes retrying after a timeout safe rather than reckless.
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