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rate-limiting

Enforce hourly and per-domain email send limits with exponential backoff retry logic.

2|Updated Mar 30, 2026
Search Tags:#email deliverability#sender reputation#email rate limiting#ISP throttling#421 errors#send throttling#per-domain limits
One-click install
npx skills add https://github.com/chunkydotdev/email-skills --skill rate-limiting-chunkydotdev
Or copy as Structured Prompt for Agent▼
Please help me install this Agent Skill.
Skill: rate-limiting
Source: https://github.com/chunkydotdev/email-skills/tree/main/skills/operations/rate-limiting
Command: npx skills add https://github.com/chunkydotdev/email-skills --skill rate-limiting-chunkydotdev

SYSTEM DOCUMENTATION & REQUIREMENTS

What problem does it solve?

Unregulated email volume spikes from bugs, batch jobs, or AI agent retry loops can cause sudden sender reputation damage, leading to spam filtering, hard rejections, and blocklisting that takes weeks to recover from. This Skill provides the guidance to implement rate limits that prevent these costly reputation cliffs.

Core Features & Use Cases

  • Multi-window rate limiting: Enforce hourly, daily, monthly, and per-minute send limits to catch runaway spikes, slow-burn volume creep, and unexpected batch overages.
  • Per-domain throttling: Configure limits for individual recipient domains to avoid overwhelming corporate mail servers and triggering 421 deferrals.
  • ISP throttling response: Learn to parse 421 SMTP error codes from Gmail, Microsoft, and Yahoo, and implement correct exponential backoff retry logic to avoid escalation to hard blocks.
  • Use Case: If your AI agent is sending 200 outreach emails and hits a retry loop that would generate 30,000 sends in an hour, use this Skill to set hourly and per-domain limits that block the runaway sends before they damage your domain reputation.

Quick Start

Use the rate-limiting skill to configure hourly and per-domain send limits for your new AI agent email outreach workflow to prevent 421 throttling errors from Gmail.

Frequently Asked Questions about rate-limiting

High-intent search queries and answers about installing and using this skill.

FAQPage Schema
How do I prevent email rate limiting and 421 deferrals from damaging my sender reputation?▼

Email rate limiting prevents sender reputation damage by enforcing hourly, daily, and per-domain send limits. This controls volume spikes from bugs or batch jobs, avoiding ISP 421 deferrals and hard rejections that cause long-term deliverability issues.

Why does Gmail throttle my emails with a 421 error during batch sends?▼

Gmail returns 421 SMTP errors when uncontrolled email volume spikes overwhelm ISP servers. Implementing per-domain throttling and exponential backoff retry logic manages send flow, preventing escalation to permanent hard blocks.

How do I stop an AI agent retry loop from sending thousands of outreach emails?▼

Stop AI agent retry loops by applying sliding window counters and token bucket rate control to enforce strict hourly limits. This blocks runaway sends before they trigger ISP throttling and damage your domain reputation.

Can I set per-domain email send limits to avoid overwhelming corporate mail servers?▼

Yes, you can configure per-domain throttling rules to set individual recipient domain limits. This prevents overwhelming corporate mail servers and helps maintain deliverability by aligning send volume with ISP requirements.

What is the best way to implement exponential backoff for ISP 421 SMTP errors?▼

The best way to handle ISP 421 SMTP errors is implementing exponential backoff retry logic combined with multi-window rate limiting. This approach parses error codes from providers like Yahoo and Microsoft to avoid escalation to blocklisting.

Do I need multi-window send limits to catch slow-burn volume creep?▼

Yes, multi-window send limits are necessary to catch slow-burn volume creep. Enforcing per-minute, hourly, daily, and monthly limits provides layered protection against unexpected batch overages and sudden volume spikes.

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