How does Microsoft throttle senders differently than Gmail?

Microsoft throttles senders differently than Gmail by applying stricter rate limits based on IP reputation and volume patterns. New or low reputation IPs face aggressive throttling even at modest volumes. Microsoft's limits often surprise senders accustomed to Gmail's more gradual approach.

Microsoft's throttling manifests through 421 errors requesting retry. The system expects senders to respect these signals and slow down. Ignoring them or retrying too aggressively can escalate to harder blocks. Proper sending infrastructure should automatically reduce speed when receiving 421 responses.

The thresholds vary based on IP reputation, with established good senders enjoying higher limits. Warming up new IPs at Microsoft requires particular patience, often taking longer than at other providers. Volume increases should be gradual, monitoring for throttling signals before pushing higher.