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.