What is Email Bounce?

An email bounceis a message returned to the sender because it couldn’t be delivered. Instead of reaching the recipient, the sending server gets back a non-delivery report (often called a bounce message or NDR) explaining what went wrong.

Hard vs. soft bounces

• Hard bounce— a permanent failure. The address doesn’t exist, the domain is invalid, or the server has explicitly rejected it. You should stop mailing that address; repeated hard bounces hurt your sender reputation.
• Soft bounce — a temporary failure. The mailbox is full, the server is down, or the message is too large. Senders typically retry for a while before giving up.

Why bounces happen — and why they matter

Bounces are generated at the SMTPlayer when a server can’t accept a message. Common causes include typo’d addresses, expired mailboxes, and authentication failures — a message that fails DMARC under a p=rejectpolicy can be bounced outright. High bounce rates signal to mailbox providers that you’re mailing stale or invalid lists, which drags down deliverability for your good mail too. Keeping bounce rates low is a core part of list hygiene.