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Deliverability

Throttling

The practice of limiting email sending speed to avoid triggering spam filters or overwhelming recipient servers.

Definition

Throttling is the practice of intentionally limiting the rate at which emails are sent to a particular recipient server or overall. This prevents overwhelming receiving servers, which might block or delay messages if too many arrive too quickly. Throttling is especially important for large sends and new sending IPs.

Why It Matters

Sending too fast can trigger rate limiting, temporary blocks, or deliverability problems. Recipient servers may reject or defer messages that arrive too quickly from an unfamiliar sender. Throttling builds trust with ISPs and ensures more reliable delivery.

How It Works

Your ESP or sending infrastructure limits the number of emails sent per time period to specific domains or overall. Limits may be automatic based on best practices or configurable. During IP warming, strict throttling gradually increases as reputation builds.

Best Practices

  • 1Let your ESP handle automatic throttling for most sends
  • 2Be especially careful with large sends to single domains
  • 3Throttle more conservatively for new IPs during warming
  • 4Monitor deferrals and adjust throttling if needed

Frequently Asked Questions

It depends on your sender reputation and receiving servers' policies. Established senders can send faster. New IPs should start slowly (hundreds per hour) and increase over weeks. Your ESP typically manages appropriate throttling.

Slightly, for large campaigns. A campaign to 100,000 subscribers might take minutes to hours to complete depending on throttling. The delay is worthwhile to ensure better deliverability.