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.
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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: publishes no per-IP limit. It tracks volume and feedback per domain and IP, and rejects mail over quota with SMTP error 4.7.28. The 2,000-a-day figure you'll see quoted is Google Workspace's per-user sending limit, not a receiving limit.
- Microsoft: similar pattern, more aggressive on IP-reputation-based throttling
- Yahoo: accepts a limited number of messages per SMTP connection and publishes no numbers
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, a little at a time, and watch inbox placement before each step up
- 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
Track real engagement, not Apple MPP noise.
Start Free TrialFrequently asked questions
Does Outsolvi affect throttling?+
Only for the mail it sends for you. Follow-ups and sequence steps go out through your own mailbox, so your provider's limits apply to them, and sequences add a per-mailbox daily cap, 50 by default. Throttling for the rest of your mail is controlled by your sending platform (Outlook, Gmail, Saleshandy, etc.).
How fast can I ramp up sending?+
No mailbox provider publishes a safe ramp. Increase volume gradually, watch deferrals and bounces, and slow down at the first sign of throttling.
Put this concept into practice
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Related glossary terms
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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.