technical · sourced answer
Hard Bounce vs Soft Bounce: Technical Definitions
A hard bounce is a permanent delivery failure caused by an invalid email address or non existent domain. A soft bounce is a temporary delivery failure caused by a full mailbox, server downtime, or message size limits. Hard bounces require immediate removal of the address from the mailing list to protect sender reputation.
Hard Bounce Definition
A hard bounce occurs when an SMTP server returns a permanent failure code, typically in the 5xx range. This indicates that the destination address is invalid, the domain does not exist, or the receiving server has blocked the sender permanently. These failures are non recoverable and signify that the email can never be delivered to that specific recipient.
Soft Bounce Definition
A soft bounce is a transient failure indicated by 4xx SMTP response codes. This means the email address is valid, but the message cannot be delivered at this moment. Common causes include the recipient mailbox being over quota, the receiving server being temporarily unavailable, or the message exceeding the maximum size allowed by the destination server.
Mechanical Process
When a sending server attempts to deliver a message, it engages in an SMTP handshake. If the receiving server rejects the message, it sends a bounce message containing a status code. Hard bounces trigger a permanent fail state in the sending application. Soft bounces often trigger a retry schedule where the sending server attempts delivery again at intervals before eventually marking it as a hard bounce.
Impact on Sender Reputation
High hard bounce rates signal to ISPs that a sender is using outdated or purchased lists, which can lead to IP blacklisting. Soft bounces have less immediate impact but excessive soft bounces can still degrade reputation. Maintaining a clean list using SendHQ free tools helps minimize these failures and improves overall deliverability metrics.
Operational Example
Example 1: A user types user@nonexistent-domain.com. The DNS lookup fails or the SMTP server returns 550 User unknown. This is a hard bounce. Example 2: A user has 15GB of mail and their inbox is full. The server returns 452 Requested action not taken: mailbox full. This is a soft bounce.
Questions teams ask
Should I remove soft bounces from my list?
Not immediately. Most systems retry soft bounces for several days. Only remove them if they consistently bounce over a long period, effectively becoming hard bounces.
Which SMTP codes indicate a hard bounce?
Hard bounces are generally associated with 5xx series codes, such as 550 (User unknown) or 554 (Transaction failed).
Can a soft bounce become a hard bounce?
Yes. If a message continues to soft bounce due to a full mailbox and exceeds the retry limit of the sending server, it is typically categorized as a hard bounce.
Primary sources
- RFC 5321: Simple Mail Transfer Protocol — RFC Editor
- M3AAWG Best Common Practices — M3AAWG