Glossary
What is email bounce rate?
The share of sent emails returned as undeliverable, split into permanent hard bounces and temporary soft bounces.
Definition
Bounce rate in email is the percentage of sent emails that come back as undeliverable, calculated as bounced emails divided by emails sent, times 100. Bounces are either hard, meaning a permanent failure such as an address that doesn't exist, or soft, meaning a temporary problem such as a full mailbox.
Hard vs soft bounces
- Hard bounce. A permanent failure: the address or domain doesn't exist, or the server refuses the message for good. In SMTP terms, this is a 5xx reply, which RFC 5321 says the sender should not simply retry. Remove these addresses right away.
- Soft bounce. A temporary failure: a full mailbox, a server that is down, or a message that is too large. These are 4xx replies, which RFC 5321 treats as temporary, so the message can be tried again. If an address soft-bounces repeatedly, stop sending to it.
How to calculate it
Say you send a cold email sequence to 500 contacts and 15 messages bounce: 11 hard, 4 soft. Your bounce rate is 15 / 500 = 3%, with a 2.2% hard bounce rate. Track hard bounces separately, because they point to bad data, while soft bounces are often out of your hands. These numbers are illustrative.
Why it matters
A high bounce rate usually means a stale, bought or unverified list. Repeatedly sending to addresses that don't exist can hurt your sender reputation, which affects whether your other emails reach the inbox. Many email platforms also pause or review accounts with high bounces, and each sets its own limit, so check your provider's policy. Track bounce rate per campaign and per list source, so you can drop the data source that produces the most bad addresses.
Email bounce rate vs website bounce rate
The same phrase means something different in web analytics: the share of visits where someone leaves without engaging. The two are unrelated. When a sales team says "bounce rate", they almost always mean returned emails.
How Siela AI uses it
Siela verifies email addresses before sending outreach, which helps keep bounces down.
Questions
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure, such as an address or domain that doesn't exist, and you should remove that address. A soft bounce is temporary, such as a full mailbox or a server outage, so the email may get through on a later attempt.
How do I reduce my email bounce rate?
Verify addresses before sending, especially on purchased or scraped lists. Remove hard bounces immediately, suppress addresses that soft-bounce repeatedly, and re-check old contacts before a new campaign, since people change jobs and their work addresses stop working.
Sources, checked 25 October 2026: RFC 5321: Simple Mail Transfer Protocol (section 4.2.1, reply codes).
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