Why do mailbox providers throttle emails?

Mailbox providers aren't being obstructive when they throttle your emails. They're doing their job, which is to manage the flow of billions of messages a day without melting their infrastructure or letting spammers through.

There are a few distinct reasons they slow things down:

Infrastructure protection. Even Gmail can't absorb unlimited traffic from a single sender at unlimited speed. Throttling keeps their inbound queues from getting overwhelmed. If you send 2 million messages at once, they'll spread that out over hours. It's a practical necessity, not a judgment on your quality.

Reputation evaluation. When they don't know you well, providers slow your messages down while they observe your behavior. A new IP or domain sending at high volume triggers a "let's watch this one" response. The sending reputation you build over time determines how much benefit of the doubt they extend. Established senders with clean histories typically sail through. New senders get watched more closely.

Inconsistency signals. Sending patterns that jump around (sending nothing for months, then blasting a huge campaign) look suspicious. Providers are more comfortable with consistent, predictable sending volumes. Sudden spikes from unfamiliar senders are one of the more reliable signals that something sketchy might be happening.

Risk management on their end. If a sender's complaint rate starts climbing or bounce rates spike mid-send, throttling gives providers time to assess the situation without fully committing to accepting the whole batch. It's a defensive pause button.

Throttling usually isn't a red flag if you're a new sender doing an IP warm-up. It becomes a concern when it's happening to an established sender with a history, or when deferrals are unusually long. If you're seeing persistent throttle behavior and you can't figure out why, checking your authentication setup and recent complaint rates is usually the right first step.