Where you saw it: Queues backing up against outlook.com and mail.protection.outlook.com, 421 responses in mail logs, or a verification run where Microsoft domains return mostly unknown.
Why it happens
- A new sending IP with no history at Microsoft, which is throttled hardest of all.
- Too many simultaneous connections, which Microsoft caps at a low number per sender.
- Poor or unknown IP reputation in SNDS.
- A sudden volume increase, which reads as a compromised source.
- High complaint rate, which tightens the throttle rather than triggering a block.
The fix, in order
Enroll in SNDS and read your own data
Smart Network Data Services shows complaint rates and filter results per IP. It needs IP ownership rather than a domain, so a shared platform may not qualify you.
Cut concurrent connections hard
Microsoft is much stricter about parallel connections than Google. Two or three per destination is often the right number, and ten will make throttling worse.
Spread the send over more hours
The same volume delivered over eight hours instead of one usually clears without any other change.
Retry with proper backoff
A 421 must be retried on an increasing schedule. Retrying immediately at the same rate keeps you throttled.
Warm up a new IP before a real campaign
Microsoft's throttle relaxes with consistent low-complaint sending over weeks. There is no way to shortcut it.
Join the Junk Mail Reporting Program
It is Microsoft's feedback loop, and it is the only way to see complaints from their consumer domains.
How to know it worked
The short version
- Queue depth against Microsoft domains clears within your normal window.
- 421 responses fall as a share of connection attempts.
- SNDS shows green status and a complaint rate under 0.3%.
Questions people ask
Why is Microsoft stricter than Gmail about connections?
Different architecture and different defaults. Exchange Online Protection limits concurrency aggressively, while Google is more tolerant of parallel connections and stricter about engagement.
How does this affect verification?
The same throttle applies to probes. A batch of Microsoft-hosted addresses returns a higher share of unknowns, which we report as unknown and do not bill rather than converting a 421 into a verdict.
How long does warming up take at Microsoft?
Two to four weeks of consistent sending with low complaints. There is no request that speeds it up.