Hard Bounce vs Soft Bounce: What's the Difference?
· 5 min
What is a hard bounce?
A hard bounce is a permanent delivery failure. The recipient's address does not exist, the domain has no mail server, or the server has permanently rejected your message. Hard bounces should be removed from your list immediately and never mailed again. Continuing to send to them signals to inbox providers that you're not maintaining your list.
What is a soft bounce?
A soft bounce is a temporary delivery failure. The mailbox is full, the server is temporarily unavailable, or the message was too large. Most email service providers automatically retry soft bounces several times over 24–72 hours. If a soft bounce persists beyond 3–5 attempts, treat it as a hard bounce and suppress the address.
Common SMTP bounce codes
550 (Requested action not taken: mailbox unavailable) and 551 (User not local) are hard bounces. 421 (Service temporarily unavailable) and 452 (Mailbox full) are soft bounces. Your email service provider translates these codes into "hard" and "soft" labels in your campaign reports.
The impact on deliverability
A bounce rate above 2% will cause most ESPs to warn or suspend your account. A rate above 5% signals to inbox providers that you're sending to poor-quality lists, reducing your chances of reaching the inbox even for valid addresses. Keep hard bounces at 0% (suppress immediately) and soft bounces below 1%.
Prevention with email validation
The best way to prevent hard bounces is to verify email addresses before adding them to your list. A good verifier checks syntax, DNS and MX records, and then confirms with the recipient's mail server that the mailbox exists — without sending a real message. Catching invalid addresses at the point of entry eliminates most hard bounces before they can damage your reputation.