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Deliverability

Bounce

An email that could not be delivered and is returned to the sender with an error message.

Definition

An email bounce occurs when a message cannot be delivered to the recipient's email address. The receiving server rejects the email and sends a bounce message (also called a Non-Delivery Report or NDR) back to the sender explaining why delivery failed. Bounces are classified as either hard bounces (permanent failures) or soft bounces (temporary issues). Understanding bounces is essential for maintaining list quality and sender reputation.

Why It Matters

High bounce rates damage your sender reputation and can get you blocklisted by email providers. ISPs view high bounces as a sign of poor list quality or spammy behavior. Managing bounces promptly is essential for maintaining deliverability. Google requires bounce rates under 2% for bulk senders.

How It Works

When an email cannot be delivered, the receiving mail server generates a bounce message containing an error code and description. SMTP error codes starting with 5xx indicate permanent failures (hard bounces), while 4xx codes indicate temporary issues (soft bounces). Your email service provider receives this bounce and should automatically categorize it and update the subscriber's status.

Example

You send a campaign to 10,000 subscribers. The delivery report shows:

  • 9,500 delivered
  • 300 soft bounces (mailbox full or server busy)
  • 200 hard bounces (invalid addresses)

Your bounce rate is 5% - which is too high.

Looking at the hard bounces, you see many are from a list you imported last month without verification. You remove all hard bounces immediately and verify the remaining list.

Soft bounces are retried over 48 hours - most eventually deliver. After cleanup, your next campaign has only 0.5% bounces.

Best Practices

  • 1Remove hard bounces from your list immediately after the first occurrence
  • 2Monitor soft bounces and remove after 3-5 consecutive failures
  • 3Use email verification before sending to new or imported addresses
  • 4Keep overall bounce rates below 2% to maintain good reputation
  • 5Check bounce reasons to identify patterns (old list, fake signups, etc.)

Automatic Bounce Handling

Sequenzy automatically processes bounces, categorizes them, and updates subscriber statuses to protect your reputation.

Learn More

Frequently Asked Questions

A hard bounce is a permanent delivery failure (invalid address, domain does not exist, mailbox does not exist). A soft bounce is temporary (mailbox full, server down, message too large). Hard bounces should be removed immediately; soft bounces may succeed on retry.

Keep your bounce rate under 2% for good deliverability, ideally under 1%. Above 2% will harm your reputation with Gmail and other providers. Above 5% is problematic, and some ESPs may suspend accounts with bounce rates above 10%.

Valid-looking addresses can bounce for several reasons: the person left the company and their mailbox was deleted, the mailbox is full and not monitored, the server is temporarily down, your IP is blocked by their provider, or the address was valid when collected but has since been deactivated.