Hard vs Soft Bounce
Hard Bounce vs Soft Bounce: The Basic Difference
Hard bounce vs soft bounce is the basic distinction between two kinds of failed email delivery. A hard bounce is a permanent failure, such as an address that does not exist. A soft bounce is temporary, such as a full mailbox or a server that is briefly unavailable.
When a receiving server refuses a message, the sending server usually generates a bounce message, also called a non-delivery report (NDR). It is sent back to the envelope sender, the address the message was actually routed from. The report includes an SMTP reply code that shows whether the failure is permanent or temporary. Those codes are explained in SMTP error codes explained.
Why the Two Types Need Different Responses
A hard bounce usually means the mailbox will never accept mail. Retrying it wastes resources, and repeated attempts to unknown addresses are a sign of poor list quality to mailbox providers. Hard bounces are normally removed from the sending list.
A soft bounce is different. The receiving server may accept the message later, so the sending server keeps retrying for a set period before giving up. A common cause is a full mailbox, which is covered in email storage quotas and attachment limits. If the problem persists across many sends, a soft bounce is usually treated as a hard one.
Bounce rates matter for sender reputation, because a high share of failed addresses suggests that mail is going to outdated or unchecked lists.
A Bounce Pattern in a Newsletter List
A small shop sends a newsletter to about 2,000 customers and finds around 200 bounces the next morning. Most are hard bounces saying the user does not exist, which usually means customers changed addresses or closed their accounts. Those addresses are removed. A smaller group of soft bounces reports a full mailbox. The sending server retries these for several days, and most are delivered. The shop keeps those addresses and watches them over later sends.
Frequently asked questions
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure, caused by a reason that will not change, such as a nonexistent address or a domain that does not receive mail. A soft bounce is temporary and may succeed on a later attempt, for example when a mailbox is full or a server is busy.
Should I keep sending to addresses that soft bounce?
Usually for a limited time. Sending servers retry soft bounces for a period, and many senders only treat an address as invalid after repeated soft bounces across several sends. Addresses that keep failing should be removed, because continued attempts add to bounce rates and can affect reputation.