Email throttling is when a receiving server slows down how many emails it accepts from a sender. It answers with a temporary "try again later" (a 4xx code) instead of accepting the message, and your server retries.
Why providers throttle
- A new domain or IP suddenly sending a lot.
- A sender whose reputation is slipping.
- Too many emails at once to the same provider.
Throttling is a warning: if the pattern continues, the next step is the spam folder or rejection.
Throttling on your side
Good senders throttle themselves first: a daily limit per mailbox, a gap of a few minutes between emails, and sending spread across the working day instead of in one burst. That looks like a person sending email, which is what providers expect from a business mailbox.
For cold email
Keep each mailbox to around 30 cold emails a day, add mailboxes rather than volume, and ramp new mailboxes up slowly. Work out how many mailboxes your goal needs with the free mailbox calculator.