What is message throttling and how to detect it?

Message throttling is when a receiving server limits how fast it accepts mail from you. It's a form of rate limiting that slows delivery without outright blocking.

Why providers throttle:

Protect their infrastructure from overload

Manage suspicious senders without blocking legitimate mail

Enforce sending limits based on reputation

Control traffic during high-volume periods

How to detect throttling:

Deferral messages: Look for 4xx responses mentioning rate limits:

"421 Too many connections"

"450 Please try again later"

"452 Too many messages"

Growing queues: Messages to specific domains accumulating faster than delivering

Inconsistent timing: Some messages deliver quickly, others take hours to the same domain

ESP metrics: Dashboard showing high deferral rates to specific providers

What to do when throttled:

Reduce sending speed to the throttling domain

Spread sends over longer time periods

Check reputation (throttling often indicates reputation concerns)

Review recent changes that might have affected reputation

For persistent throttling, consider reaching out to the provider's postmaster team

Throttling is a warning sign, not a full block. It often precedes more serious action if the underlying issue isn't addressed.