SkipBounce

14 August 2026 · 6 min read

Why emails bounce — and what it costs you

Every email you send to a dead address does more than fail. It tells Gmail, Outlook, and every inbox provider in between that you don't know your own list — and they remember.

The two kinds of bounces

Hard bounces are permanent failures. The mailbox doesn't exist, the domain is dead, or the server explicitly rejected the address (550 5.1.1 User unknown). Hard bounces should never happen twice: any address that hard-bounces must leave your list immediately.

Soft bounces are temporary: a full mailbox, a server timeout, greylisting. One soft bounce means retry later. An address that soft-bounces for weeks is a hard bounce wearing a disguise.

What a dirty list actually costs

  1. Inbox placement. Providers track your bounce rate per sending domain and IP. Above roughly 2%, your mail starts landing in spam — for everyone on your list, including your best customers.
  2. Real money. ESPs price by list size. If 9% of your list is undeliverable (a typical number for a two-year-old list), you're paying your ESP a premium to damage your own reputation.
  3. Spam traps. Providers convert long-dead addresses into traps. One trap hit can put your domain on a blacklist that takes weeks to escape.

Where dead addresses come from

  • Typos at signup — ola@gamil.com was never going to work
  • Churn — people change jobs; B2B addresses die at ~2–3% per month
  • Disposable addresses — temp inboxes used to grab a discount
  • Bots — fake signups that pad your list and your bill

The fix is boring and effective

Validate at the front door and clean regularly:

  1. Validate at signup with an API call — catch typos while the user can still fix them.
  2. Clean before big sends. A pre-campaign validation pass keeps your bounce rate under the radar.
  3. Suppress, don't delete. Keep undeliverable addresses in a suppression list so re-imports can't resurrect them.

SkipBounce does all three: a real-time API, bulk cleaning with CSV export, and one-click Klaviyo suppression. The first 50 checks are free — no credit card, and your data auto-deletes after 48 hours.

Why emails bounce — and what it costs you · SkipBounce