What Is Tenant Throttling?
Microsoft 365 has limits at the tenant level, not just the mailbox level. Your tenant includes all mailboxes across all your domains on one Microsoft account.
When you exceed tenant limits, Microsoft throttles ALL sending - every domain, every mailbox.
Recognizing Throttling Errors
Common bounce messages when throttled:
``
452 4.5.3 Too many recipients
421 4.7.0 Too many connections
450 4.7.1 Client was not authenticated
``
These are temporary failures - Microsoft is telling you to slow down.
Microsoft's Limits
| Limit Type | Threshold | Reset |
|---|---|---|
| Messages per minute | 30 | Per minute |
| Recipients per day | 10,000 | 24 hours |
| Recipients per message | 500 | Per message |
Why Throttling Happens
Common causes:
- Warmup too fast - Exceeding daily ramp schedule
- Multiple campaigns - Different tools sending simultaneously
- Large recipient lists - Many BCC recipients per email
- Burst sending - All emails at once vs spread throughout day
How to Fix Throttling
Immediate steps:
- Pause all sending - Let the tenant cool down
- Wait 24 hours - Limits reset daily
- Check all sending sources - Are multiple tools sending?
Long-term fixes:
- Spread sending over time - Use scheduling to avoid bursts
- Reduce daily volume - Stay well under limits
- Multiple tenants - Separate high-volume clients
Our Approach
We prevent throttling by:
- Conservative daily limits - 20 emails per domain, not maximum
- Time-spread sending - Emails distributed across business hours
- Monitoring - Detect throttling early and pause
Key Takeaway
Tenant throttling is Microsoft saying "slow down" - not "you're blocked." Respect the limits, spread your sending, and throttling resolves itself within 24 hours.