What is a soft bounce? (Email glossary definition)

Soft bounce is a temporary email delivery failure. The receiving server has rejected the message with a 4xx status code, indicating something is preventing delivery right now but may not later. Standard practice is to retry on a schedule and treat the address as hard-bounced if it fails repeatedly.

Common causes:

• Mailbox is over quota (full).
• Receiving server temporarily unavailable.
• Greylisting (server intentionally delays first delivery from unknown senders).
• Recipient mail provider rate-limiting your sending IP.
• Temporary block on specific content.

Standard ESP behavior: retry the same address 2-3 times over 24 hours, then treat as hard-bounced and add to the suppression list. The exact retry schedule varies by ESP.

Soft bounces count toward your overall bounce rate. A soft-bounce rate above 2% indicates either a sending-infrastructure issue (throttling, IP reputation) or a list with many over-quota mailboxes. See how to handle soft vs hard bounces.