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