What Is an Email Bounce (Hard vs Soft)?
An email bounce occurs when an email cannot be delivered to the recipient's mailbox. Bounces are classified as hard (permanent) or soft (temporary), and both types impact your sender reputation and deliverability.
How Email Bounces Work
When an SMTP server attempts to deliver an email and the receiving server cannot accept it, it returns a bounce message (also called a Non-Delivery Report or NDR) to the sender. The bounce includes an SMTP status code that indicates the reason for failure.
Hard bounces (5xx codes) are permanent failures. The address does not exist, the domain is invalid, or the server has permanently blocked delivery. These addresses should be immediately removed from your mailing list.
Soft bounces (4xx codes) are temporary failures. The mailbox might be full, the server temporarily unavailable, or the message too large. The sending server typically retries delivery for several days before giving up.
Why Email Bounces Matter
Bounce rates directly impact your sender reputation. High bounce rates signal to mailbox providers that you are sending to invalid addresses, which is a characteristic of spam. This can cause your legitimate emails to be filtered to spam or blocked entirely. Maintaining clean email lists and verifying addresses before sending is essential for deliverability.
Frequently Asked Questions
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent failure — the address does not exist or delivery is permanently blocked. A soft bounce is temporary — the mailbox is full or the server is temporarily down. Hard bounces require immediate list removal; soft bounces may resolve on retry.
What is a good bounce rate?
Below 2% is generally acceptable. Rates above 5% indicate list hygiene problems and can damage your sender reputation. Regularly cleaning your email list helps maintain a low bounce rate.
How do bounces affect sender reputation?
High bounce rates damage your IP and domain reputation with mailbox providers, causing more of your legitimate email to land in spam or be rejected. ISPs track bounce rates as a key deliverability metric.