Email Bounce Rate: Hard vs Soft Bounces and What to Do About Each

What Is a Bounce?

An email bounce occurs when the receiving mail server rejects your email instead of accepting it for delivery. The server sends back an error code explaining why the email was rejected.

Bounces are divided into two categories, and how you handle each one determines whether your list stays clean or your sender reputation degrades.

Hard Bounces: Permanent Failures

A hard bounce means the email address is permanently undeliverable. The address doesn't exist, the domain doesn't exist, or the server has permanently blocked you.

Common causes:

• Typos in the email address (gmial.com instead of gmail.com)
• The recipient deleted their account
• The domain no longer exists
• The address was fake or disposable

What to do: Remove the address from your list immediately. Never retry a hard bounce. Add it to your suppression list so it's never emailed again, even if re-imported.

Soft Bounces: Temporary Failures

A soft bounce means the email couldn't be delivered right now, but the address itself is valid. The problem may resolve on its own.

Common causes:

• Recipient's mailbox is full
• Recipient's mail server is temporarily down
• Your email is too large
• Rate limiting — the server is deferring your messages temporarily

What to do: Retry delivery up to 3 times over 24-72 hours. If the email still can't be delivered after 3 attempts, treat it as a hard bounce and suppress the address.

Bounce Rate Thresholds

Different providers have different tolerance levels:

• Best practice — Below 1%
• Acceptable — 1-2%
• Warning zone — 2-5% (reputation damage begins)
• Danger — Above 5% (Amazon SES suspends, other ESPs will flag your account)

A consistently high bounce rate tells mailbox providers that you don't maintain a clean list — and list quality is a major trust signal.

How to Reduce Your Bounce Rate

Before sending

1. Validate email addresses at signup — Use real-time email validation on your signup forms to catch typos and invalid addresses before they enter your list.
2. Use double opt-in — Require new subscribers to confirm their email. This eliminates typos, fake addresses, and disposable emails.
3. Run your list through a validation service — Before any major campaign, validate your full list. Services can identify invalid, disposable, and risky addresses.

During sending

4. Process bounces in real-time — Configure your ESP to suppress hard bounces immediately and track soft bounce patterns.
5. Monitor per-campaign bounce rate — A sudden spike on a specific campaign suggests a problem with that segment.

Ongoing maintenance

6. Implement a sunset policy — Automatically stop emailing addresses that haven't engaged in 90-180 days. Dormant addresses become invalid over time.
7. Re-validate periodically — Email addresses go stale. Run quarterly validation against your full list to catch addresses that have become invalid.

Reading Bounce Codes

When a bounce occurs, the receiving server returns an SMTP status code. Understanding these helps diagnose issues:

• 550 — "User not found" — Hard bounce. Address doesn't exist.
• 551 — "User not local" — Hard bounce. Server doesn't handle this address.
• 552 — "Mailbox full" — Soft bounce. Retry later.
• 553 — "Mailbox name invalid" — Hard bounce. Address format is wrong.
• 450 — "Mailbox unavailable" — Soft bounce. Temporary issue.
• 421 — "Service not available" — Soft bounce. Server overloaded or in maintenance.

Frequently Asked Questions