Email bounce rate
Email bounce rate is the percentage of sent emails that are returned undelivered. Hard bounces are permanent (invalid or non-existent address); soft bounces are temporary (full mailbox, server down). A high bounce rate signals a poor list and damages sender reputation.
How it works
When a mail server can't deliver a message it returns a bounce code. Hard bounces should be suppressed immediately; repeated sends to invalid addresses tell providers you don't maintain your list.
Why it matters
Bounce rate is a direct deliverability signal. Most providers want cold-outreach bounce rates well under ~3-5%; above that, placement drops and accounts get flagged.
How is bounce rate calculated, and which denominator should you use?
Divide bounced messages by messages attempted over the same period. In a hypothetical week with 2,000 sends, 46 hard bounces and 18 soft bounces, the hard-bounce rate is 2.3% and the combined rate is 3.2%. Report the two separately, because a soft bounce that delivers on retry is not a list problem, while every hard bounce is.
A single account-wide figure hides the cause. Calculate the rate per list source and per sending mailbox as well. A 2% overall rate can conceal one imported segment bouncing at 11% while the rest sits under 1%, and that segment is where the fix belongs. The soft bounce versus hard bounce guide covers how retries are counted.
How do you read the code in a bounce message?
The first digit tells you the class: 5 means permanent, 4 means temporary. Enhanced status codes from RFC 3463 add detail. 5.1.1 means the mailbox does not exist, the classic hard bounce to suppress at once. 5.2.2 means the mailbox is full. Codes in the 5.7 family are security or policy refusals, which often concern the sender, not the address.
That last group changes what you do. A 5.7 rejection citing authentication or reputation is a problem with your domain or mailbox, and suppressing the contact would hide it. Log the full response text alongside the digit so you can tell a dead address from a receiver that has blocked you.
What causes a bounce spike on one campaign only?
Usually the list behind that campaign. Records bought or scraped months ago decay as people change jobs, CSV imports introduce typos, and a company that switches mail providers may start rejecting addresses that used to be accepted. Sort the bounces by recipient domain; if one domain produces most of them, that receiver is refusing you or the domain has changed its setup.
Verifying addresses before the first send removes most of this risk. The free email checker handles one-off lookups, and Autocloz's Free plan includes 1,000 email verifications a day and suppresses hard bounces automatically. For a longer checklist, see how to reduce email bounce rate.
What does a low bounce rate fail to prove about a list?
That the addresses reach people. A catch-all email domain accepts mail for any address, including ones nobody reads, so a list heavy in catch-all domains can show almost no bounces and still produce no replies. Spam traps do not bounce either; they accept mail silently and report the sender. Treat a low bounce rate as a minimum requirement, then judge the list by reply rate per source and by complaint counts.
A useful habit is to record three numbers for every new list source before scaling it: hard-bounce rate, reply rate and the share of addresses sitting on catch-all domains. A source that looks clean on the first number and weak on the other two needs a smaller test before more volume goes to it.
How Autocloz handles it
Autocloz verifies addresses before sending, auto-suppresses hard bounces, and surfaces bounce trends per mailbox so a bad list segment can't silently burn your domain.
Free tools for this
No signup required — they run in your browser.
FAQ
What is a good email bounce rate?
For cold outreach, keep hard bounces under about 3%. The reliable way to get there is verifying every address before the first send and suppressing bounces automatically.
Related terms
A spam trap is an email address operated by mailbox providers and blocklist operators specifically to catch senders with poor list hygiene. There's no real person behind it, so any mail it receives indicates you're emailing addresses you didn't earn permission to contact.
Email verification is the process of checking whether an email address is valid and safe to send to — confirming the syntax, domain and mailbox exist and aren't a known trap or disposable address — before you add it to a campaign. It is the single most effective way to control bounce rate.
A hard bounce is a permanent email delivery failure — the address is invalid, the domain doesn't exist, or the mailbox was deleted — and should be suppressed immediately. A soft bounce is a temporary failure — a full mailbox, a message too large, or a server briefly down — which may succeed on a later retry.
List hygiene is the ongoing practice of keeping an email list clean and accurate — verifying addresses, removing invalid and hard-bounced contacts, suppressing unsubscribes and long-term non-engagers, and avoiding purchased or scraped data. Good hygiene protects deliverability and keeps sender reputation healthy.
Want this applied to your setup?
This is a reference page. If you have a specific question about your own outreach, leave your email and a human on our team will reply.