Deferral and Blocking
Temporary deferrals, rate limits, and hard blocks that stop or slow delivery before foldering.
- Reading time:
- 5 min
- Difficulty:
- Intermediate
- Estimated duration:
- 5 min
- Prerequisites:
- None
- Last updated:
- 2026-07-15
- Author:
- empire
Definition
Deferral and blocking are SMTP and policy responses that delay or refuse mail because of reputation, volume, or policy violations.
Why it matters
Help operators recognize when the problem is access to the receiver rather than inbox foldering.
How it works
Receivers may return temporary errors to slow unknown senders, enforce per-IP or per-domain limits, or reject outright when lists or reputation thresholds are crossed.
Limitations
- Error text is often generic; root cause still requires reputation and auth context.
Common misconceptions
- Every 4xx means greylisting you can ignore by retrying forever.
How to verify
- Group deferral codes by destination domain and sending IP.
- Check whether volume ramps or auth failures correlate with blocks.