Email Bounce Checker
Email bounces fall into two categories: hard bounces, where delivery fails permanently because the address does not exist or the domain has been decommissioned; and soft bounces, where delivery fails temporarily due to a full mailbox or server issue. Both types hurt your deliverability, but hard bounces are significantly more damaging to your sender reputation.
ISPs monitor your bounce rate closely. When your hard bounce rate exceeds 2–5%, providers begin throttling or blocking your messages. Google and Microsoft have both published bulk sender guidelines that reference bounce rate thresholds. Identifying hard-bouncing addresses before you send — or immediately after — is essential to maintaining a healthy sender score.
ISP Deliverability's list hygiene module tracks bounces from your test sends and maintains a suppression list that automatically excludes known-bad addresses from future uploads. Combined with pre-send list validation, this creates a complete bounce prevention workflow.
Understanding bounce types
- Hard bounce: address does not exist (5xx SMTP error) — remove immediately
- Soft bounce: temporary failure (4xx) — retry up to 3 times then suppress
- Block bounce: ISP is rejecting your IP or domain — requires reputation repair
- Keep hard bounce rate below 2% to maintain sender standing with Gmail and Outlook
- Suppress all hard bounces permanently — never retry them