Glossary

Throttling

Throttling

Throttling is when mailbox providers intentionally delay or limit the rate at which they accept emails from a sender. It’s not a permanent block but more like email traffic control, used when a sender ramps up volume too fast or when the provider isn’t sure yet whether the sender is trustworthy. 

When throttling kicks in, you’ll typically see: 

  • 4xx (temporary) bounce codes like 421 or 450 
  • Slower delivery speeds 
  • Signs that the ISP is “testing” your stream before granting full access 

Throttling is most often due to: 

  • Sudden spikes in email volume 
  • Lack of IP/domain reputation (new senders especially) 
  • Inconsistent sending patterns 
  • Suspicious content or engagement signals 

How to prevent throttling: 

  • Warm up your IP gradually (a.k.a. IP conditioning) 
  • Send consistently and at predictable volumes 
  • Maintain strong list hygiene and user engagement 
  • Authenticate your mail (SPF, DKIM, DMARC) to build trust 

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