Email Throttling
Also known as: Rate Limiting, Send Rate Throttling
Email throttling is rate-limiting outbound mail to stay within the acceptance thresholds set by receiving mailbox providers. Each major provider has per-IP, per-domain, and per-recipient-domain rate limits; exceeding them triggers temporary deferrals (4xx soft bounces) or, sustained, reputation damage.
Throttling is the under-discussed half of deliverability. A sender with perfect authentication and a clean list can still hit deferrals by sending too fast.
Per-provider patterns
- Google: per-sending-IP daily limit roughly 2K/day for unknown senders, scaling to millions/day for established senders. Spikes from 100/day to 10K/day in one day trigger immediate throttling.
- Microsoft: similar pattern, more aggressive on IP-reputation-based throttling
- Yahoo: published 5K/day connection limit per IP
How throttling looks operationally
- Sustained 421 / 451 / 452 SMTP responses on a previously-clean send IP
- "Try again later" or "rate limit exceeded" in SMTP responses
- Spike in deferred queue at the sender
How to throttle correctly
- Ramp volume gradually (warmup pattern): no more than 30-40% week-over-week increase
- Spread sends across the day; avoid bursts
- Limit per-recipient-domain rate (don't blast 500 emails at the same company in 5 minutes)
- Monitor 4xx response rate; pause if it spikes
- Use per-IP throttling on dedicated infrastructure
Frequently asked questions
Does Outsolvi affect throttling?+
Outsolvi is a tracking layer, not a sender. Throttling is controlled by your sending platform (Outlook, Gmail, Saleshandy, etc.).
How fast can I ramp up sending?+
Conservative: 30% week-over-week. Aggressive: 50% week-over-week. Faster than 50% triggers throttling on most receivers.
Put this concept into practice
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We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.