Bounce
Not all delivery failures mean the same thing, and handling them identically is expensive.
A bounce is a receiving server's report that it could not, or would not, deliver your email. The word hides three different events: a hard bounce (the address is permanently bad), a soft bounce (a temporary condition), and a policy block (the receiver is refusing you, not the address).
Why it matters
Bounce classification drives the suppression list, and the failure modes are asymmetric. Retrying a hard bounce annoys no one but erodes your reputation, since repeatedly hitting dead mailboxes is spammer behavior. Suppressing on a policy block is worse: the addresses are valid, the rejection was about your IP or content, and a classifier that treats "blocked due to sending reputation" as address death quietly destroys thousands of real, deliverable subscribers in a single incident.
In practice
| Class | Example | Right response |
|---|---|---|
| Hard | 550 5.1.1 user unknown | Suppress immediately |
| Soft | 452 4.2.2 mailbox full, greylisting | Retry on a schedule; suppress only after repeated failures |
| Policy block | 554 5.7.1 blocked due to sender reputation | Fix the sending problem; do not touch the address |
The SMTP enhanced status code carries most of the signal (5.1.x is an address problem, 5.7.x is a policy problem), but real receivers are inconsistent, so serious classifiers also parse provider-specific reply text.
How Email Fast handles it
Email Fast's classifier separates the three using enhanced status codes plus provider-specific reply patterns. Hard bounces suppress instantly, soft bounces retry with backoff, and a policy block never suppresses an address: it feeds the reputation breaker instead, which slows or pauses sending toward that provider until the block clears. Bounces are matched to the exact message via VERP.