An email bounces when the receiving server refuses it, almost always because the mailbox does not exist, the domain cannot receive mail, or the mailbox is full. You can predict a bounce before sending by verifying the address against the live server, which catches the dead ones in advance. On lists that have not been checked in 90 days, BounceZero typically finds 14.5% invalid on business lists, every one of which would have bounced.
A bounce is the receiving server saying no. The common reasons are simple: the mailbox was never real or has been deleted (a hard bounce), the domain has no mail server at all, or the mailbox is full or temporarily unavailable (a soft bounce). A hard bounce will never deliver; a soft bounce might, later.
The reason to care is that bounces are counted against you. Gmail, Outlook and the rest watch your bounce rate, and once it passes about 2%, the working limit most sending platforms enforce, they start filtering your mail to spam, including the messages to good addresses. One bad list can quietly wreck a sending domain you spent months warming.
A proper check does not just say yes or no. It tells you why, so you know what to do with each address.
| Result | What it means | Will it bounce? | What to do |
|---|---|---|---|
| Valid | The mailbox exists and answered | No | Safe to send |
| Invalid | No mailbox, or the domain cannot receive mail | Yes, every time | Remove it |
| Catch-all | The server accepts everything, so it cannot be confirmed | Maybe | Send carefully, low volume |
| Unknown | The server blocked the check | Cannot be predicted | Hold or re-check later |
| Disposable / role | Throwaway domain, or info@ / sales@ | Often, or ignored | Exclude from cold sends |
The split between hard and soft bounces is explained in what is a hard bounce.
Paste it into the free checker. In under a second you get valid, invalid, catch-all or unknown, with the reason. Good for checking a single prospect before you reach out.
Upload the CSV to bulk validation. It returns the same verdict per row for up to a million addresses, so you can strip the bouncers before import.
Put the check on your form through the API, so a bouncing address never enters your list in the first place.
Addresses decay: in our study only 19.2% of addresses were still valid when re-checked more than 90 days after the last check. Re-verify an active list before every send.
Verify the address first. A verifier checks the syntax, the domain's mail records and whether the mailbox answers, all without sending, and tells you whether it would bounce. Invalid addresses bounce every time; valid ones do not; catch-all and unknown cannot be predicted and should be sent to carefully or held.
Yes, up to your free allowance. BounceZero gives every account 100 checks a month at no cost, enough to test a small list or a sample of a big one. Larger lists use credit packs that never expire, and unknown results are never charged.
Under 2% is the working limit most sending platforms enforce before they start filtering you to spam. Under 1% is healthy. The only reliable way to stay there is to verify the list before each send and remove the invalid addresses in advance.
Because addresses decay. People leave jobs, close accounts and change providers, so a list that was clean three months ago is not clean now. In our decay study only 19.2% of addresses were still valid when re-checked after more than 90 days. Re-verify before every send.
No. Verification opens a connection to the receiving server and asks whether the mailbox exists, then closes it before any message is sent. The person is never contacted, so checking never affects your sender reputation.
Check one address or a whole list before you send. Invalid, catch-all and unknown each labelled, so you remove the bouncers in advance. 100 free a month.
Ayoub built BounceZero's 5-stage validation pipeline, its dedicated sending infrastructure, and the high-availability database behind every verification. Previously built high-volume email delivery systems. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.
Single-email checks, no signup required
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Check if an address exists before you send
Syntax, MX, SMTP, catch-all and API explained
The cost of a check vs the cost of a bounce
Check if any email address is real
Instant real-time single address check
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