Bounce (Email Bounce)

What Is an Email Bounce?

An email bounce occurs when a message cannot be delivered to the recipient's inbox and is returned to the sender. The mail server at the receiving end sends back a notification — called a Non-Delivery Report (NDR) or bounce message — explaining why delivery failed. Bounces come with SMTP error codes that indicate the specific reason for failure. Understanding bounces is essential for maintaining a healthy sender reputation and effective email program.

Hard Bounce vs. Soft Bounce

There are two main types of bounces. A hard bounce is a permanent delivery failure — the address doesn't exist, the domain doesn't exist, or the recipient's server has permanently rejected the address. Hard bounces should be removed from your list immediately. A soft bounce is a temporary failure — the mailbox exists but is full, the server is temporarily down, or the message was too large. Soft bounces may resolve themselves if you retry later.

Why Bounce Rates Matter

Your bounce rate — the percentage of sent emails that bounce — is one of the most closely watched metrics by email service providers. A bounce rate above 2% is generally considered a red flag. Most major ESPs (Mailchimp, SendGrid, Klaviyo) will pause or suspend your account if your bounce rate climbs too high. Beyond ESP enforcement, high bounce rates signal to ISPs that you're not maintaining your list properly, which damages your sender reputation and can lead to deliverability problems across all your sends.

What Causes Bounces

Invalid email addresses from form typos or fake signups. Addresses that have been abandoned or deactivated since you collected them (email list decay). The recipient's mailbox being over its storage quota. The recipient's mail server being temporarily unavailable. Your sending IP or domain being blocked by the recipient's server. Each cause has a different solution, but the most preventable cause — sending to addresses that were never valid or have since become invalid — is directly addressed by email verification.

How EmailVerify.io Reduces Bounces

EmailVerify.io checks each email address against multiple layers of validation before your campaign goes out — syntax checks, domain/MX record verification, SMTP handshake testing, catch-all detection, and disposable email filtering. Addresses that would bounce are flagged and removed before they ever enter your send list. Customers routinely reduce their bounce rate to below 1% after running their lists through EmailVerify.io. Start with a free verification at emailverify.io.

Bounce Handling in Your ESP

Most ESPs handle bounces automatically — hard bounces are usually suppressed after one occurrence, soft bounces after three to five retries. Make sure your ESP's bounce handling is configured correctly and that you're not importing suppressed addresses back into active lists. Export your hard bounce list periodically and exclude those addresses from any future imports.