What is traffic shaping in email?

Traffic shaping is the broader category that throttling and greylisting both fall under. It's the set of techniques a receiving mail server uses to control how much and how fast it accepts email from a given sender.

Where throttling slows your delivery rate ("we'll take your messages, just not all at once"), traffic shaping is the policy layer that decides when to throttle, how much, and based on what signals. Think of throttling as one tool in the traffic shaping toolkit.

The main inputs a provider uses when shaping traffic from you:

Sending volume and velocity. How many messages are you sending, and how fast? A sudden jump from 1,000 messages a day to 500,000 looks suspicious. Gradual increases look intentional. Providers shape traffic more aggressively when volume jumps unexpectedly.

Sender reputation. Your IP and domain reputation are the main variables. High reputation senders get a wider lane and faster acceptance. Lower-reputation senders get squeezed down to a trickle until the provider builds enough confidence to let more through.

Historical behavior. Senders with a consistent history get more leeway. New IPs and domains, or accounts with spotty complaint or bounce histories, get tighter shaping applied while providers evaluate them.

For most senders using an established ESP, traffic shaping happens in the background and you'll never notice it. It only becomes visible when delivery slows noticeably or when you're warming up new infrastructure.

And if you're seeing shaping-related delays, the usual fixes apply: validate your authentication, keep your bounce and complaint rates low, and build volume gradually rather than in spikes. If things are actively broken, the SOS hotline is a free way to get a second pair of eyes on your setup.