Hard bounce vs. soft bounce: what's the difference and why it matters
Not all bounces are equal. Hard bounces signal a permanent problem; soft bounces are temporary. Treating them the same way is one of the most common deliverability mistakes we see.
Your ESP's bounce report is one of the most important signals you have about your list health — but only if you know how to read it. The distinction between hard and soft bounces is fundamental, and handling them differently is non-negotiable.
Hard bounces
A hard bounce is a permanent delivery failure. The most common causes are: the email address doesn't exist (550 5.1.1 User unknown), the domain doesn't exist (NXDOMAIN), or the receiving server has permanently blocked your messages. Hard bounces should be removed from your list immediately after they occur — no exceptions. Continuing to send to them is the fastest way to damage your sender reputation.
Soft bounces
A soft bounce is a temporary delivery failure. Common causes include: the recipient's mailbox is full, the receiving server is temporarily unavailable, or the message exceeded the size limit. Soft bounces should be retried — most ESPs do this automatically. If an address soft-bounces repeatedly over 5–7 days without successful delivery, treat it as a hard bounce and suppress it.
Most ESPs define 3–5 consecutive soft bounces as effectively equivalent to a hard bounce. Check your platform's policy and configure suppression accordingly.
Where email validation fits in
Email validation prevents hard bounces before they happen. An SMTP probe on an invalid address at sign-up time catches the same condition your ESP would encounter during delivery — except it catches it before the address enters your list and before you spend a credit sending to it.
Bounce rate targets
- Hard bounce rate: keep under 0.5% per campaign
- Soft bounce rate: keep under 2% per campaign
- If hard bounce rate exceeds 2%, pause sends and clean your list immediately
- Monitor trends, not just absolute numbers — a rising rate is a warning sign