Repeatedly sending to invalid addresses is one of the strongest negative reputation signals providers track.
Mistake 1: Treating 'mailbox full' as a hard bounce and losing valid contacts#
Why it hurts: this undermines email bounce handling at the point where mailbox providers make their decision, and it is rarely surfaced by your own tooling until placement drops.
Fix: Parse DSN codes: 5.1.x means the address does not exist (hard); 4.x.x means temporary (soft).
Mistake 2: Ignoring 'blocked by policy' bounces that signal a blocklist or DMARC failure#
Why it hurts: this undermines email bounce handling at the point where mailbox providers make their decision, and it is rarely surfaced by your own tooling until placement drops.
Fix: Suppress hard bounces after one failure and soft bounces after three.
Frequently asked questions#
What is the difference between a hard and soft bounce?
Hard bounces are permanent failures (bad address). Soft bounces are temporary (full mailbox, server down). Suppress hard immediately.