Email Throttling: Why It Happens and How to Fix It
Security Boulevard, Monday, October 26th, 2026
Explains why mailbox providers throttle email with temporary 4xx deferrals, how to recognize it and practical steps to fix it.
This EasyDMARC post syndicated on Security Boulevard explains that email throttling happens when a mailbox provider temporarily slows delivery and asks the sender to retry, usually via 4xx SMTP codes such as 421 or 450, which differs from hard bounces and outright blocking.
Common causes include sudden volume spikes, poor sender reputation, high bounce or complaint rates, low engagement, shared or abused IPs, SPF, DKIM or DMARC failures and too many concurrent connections.
To diagnose it, the author suggests checking response codes, deferrals in logs and whether delays affect one provider or many.
Fixes include ramping volume gradually, batching campaigns, cleaning lists, separating marketing and transactional mail and verifying authentication.