What’s a rate limit and how do ESPs manage it?

A rate limit restricts how many messages a recipient server accepts from any sender within a time period. Exceeding the limit triggers deferrals (4xx) or blocks. Limits vary by provider and by sender reputation.

Types of rate limits:

Connections per hour: Maximum simultaneous or total connections from your IP

Messages per connection: How many messages before you must reconnect

Messages per hour/day: Total volume to that provider

Recipients per message: How many addressees per single email

How ESPs manage limits:

Throttling: Automatically slow sending to stay within observed limits. Add delays between messages or connections.

Monitoring: Track deferral patterns to detect when you're hitting limits. Adjust automatically.

Reputation-based allocation: Senders with better reputation may receive higher limits from providers. ESPs route their best traffic from their highest-reputation IPs.

Spreading load: Distribute messages across multiple IPs to multiply effective limits.

Mailbox providers don't publish exact limits, and they vary by your reputation. ESPs learn limits empirically by monitoring delivery feedback.

The harbor has a maximum number of ships it can process per day. Arrive too fast and you wait at anchor.