StopBouncingEmail Bounce Checker — Stop Bounces Before You Send
Check your email list for addresses that will bounce before you hit send. StopBouncing uses live SMTP verification to identify invalid, expired, and undeliverable addresses — so bounces never reach your sender reputation.
100 free checks on signup. No credit card required.
Hard Bounce vs Soft Bounce
Not all bounces are equal. Understanding the difference determines whether you remove an address or retry it.
SMTP 550 5.1.1 — User Unknown- Mailbox does not exist (user never existed, or deleted their account)
- Domain does not exist or has no MX records
- Mail server permanently rejects the address (SMTP 550 error)
Action: Remove immediately. Never send to a hard bounce again — repeated sends to hard-bouncing addresses are the fastest way to get your domain blacklisted.
SMTP 452 / 421 — Temporary failure- Mailbox is full (inbox storage limit reached)
- Mail server temporarily unavailable or overloaded
- Message too large for the recipient's mailbox
Action: Retry after 24–48 hours. Most ESPs retry soft bounces automatically. After 3–5 failed attempts with no success, treat as a hard bounce and remove.
Why Email Addresses Bounce
The five most common causes of email bounces — and how to catch them in advance.
| Cause | Detail | Frequency |
|---|---|---|
| Address no longer exists | People leave companies, cancel email accounts, or change providers. Email lists decay at roughly 20–30% per year. An address that was valid 12 months ago may hard bounce today. | Most common |
| Typos at collection | gmial.com, hotmial.com, yaho.com — small typos at the point of entry create addresses that never existed. A bounce checker catches these before your first send. | Very common |
| Disposable email services | Temporary addresses from services like Mailinator, Temp-Mail, and Guerrilla Mail expire quickly. They may pass format checks but bounce shortly after being added to a list. | Common in B2C / signups |
| Domain changes | Companies rebrand, get acquired, or go out of business. The old domain stops receiving email entirely. Everyone at that domain hard bounces. | Common in B2B |
| Old or purchased lists | Lists older than 12 months or bought from third parties often have 15–40% invalid addresses. Running a bounce check before any send is non-negotiable for cold or stale data. | High risk |
Acceptable Bounce Rates by Industry
What counts as a safe bounce rate depends on your industry and sending type. Going above the warning threshold triggers ESP reviews.
| Industry | Acceptable | Warning | Critical |
|---|---|---|---|
| E-commerce | < 0.5% | 0.5–2% | > 2% |
| B2B SaaS | < 1% | 1–3% | > 3% |
| Media / publishing | < 0.3% | 0.3–1% | > 1% |
| Lead generation | < 2% | 2–5% | > 5% |
| Recruiting | < 1.5% | 1.5–4% | > 4% |
| Cold outreach | < 3% | 3–8% | > 8% |
How the Email Bounce Checker Works
StopBouncing identifies bounce-prone addresses using the same SMTP process your mail server uses during delivery — without sending a single message.
DNS & MX verification
Confirms the domain exists and has a properly configured mail server. Domains with no MX records hard bounce on every send.
Live SMTP handshake
Connects to the mail server and asks whether the specific mailbox exists. The server responds before any email is sent.
Disposable detection
Identifies temporary addresses from known throwaway services. These typically expire within hours or days of creation.
Email Bounce Checker FAQ
What is an email bounce checker?
An email bounce checker is a tool that identifies which email addresses in your list will bounce before you send. It does this by performing SMTP-level checks — connecting directly to the recipient's mail server to confirm whether the mailbox exists and can accept email. This lets you remove undeliverable addresses before they generate bounces that damage your sender reputation.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure — the address doesn't exist, the domain has no mail server, or the server permanently rejected it. You must remove hard bounces immediately and never send to them again. A soft bounce is a temporary failure — the inbox is full, the server was temporarily unavailable, or the message was too large. Most ESPs retry soft bounces automatically for 24–72 hours before marking them as failed.
How does StopBouncing check for email bounces?
StopBouncing performs a full SMTP verification on every address: DNS lookup to confirm the domain exists, MX record check to confirm a mail server is configured, and an SMTP handshake to the mail server to confirm the specific mailbox responds. This is the same process an email server uses when actually delivering a message — without ever sending one. Addresses that fail at any stage are flagged before you send.
How many email bounces are acceptable?
Gmail and Outlook Postmaster tools flag senders above a 0.08–0.1% spam complaint rate. For bounces specifically, most ESPs (Mailchimp, Klaviyo, SendGrid) will suspend accounts above 2% hard bounce rate. In practice, keep hard bounces below 1% to stay safe. For cold outreach, keep below 3–5% to avoid domain blacklisting.
Can I check email bounces in bulk?
Yes. Upload a CSV file with any email column to your StopBouncing dashboard. Our parser finds the email field automatically and runs bounce checks on every address. Download the results with a status column added — filter out Invalid and Disposable before you send.
How often should I run a bounce check?
Before every campaign on a list you haven't emailed in the past 3 months. Before any cold outreach. After importing contacts from a third-party source. Email addresses decay at 20–30% per year, so a list verified 6 months ago will have accumulated a meaningful number of new invalids.
Will checking for bounces harm my sender reputation?
No. StopBouncing's SMTP bounce checking uses its own verification infrastructure — your domain and sending IP are never involved. The process is equivalent to a mail server performing its own pre-delivery check, and it does not generate complaints, bounces, or any record on the recipient's side.
What happens if I ignore email bounces?
High bounce rates trigger automatic throttling and blacklisting by major inbox providers. Mailchimp suspends accounts above 2% hard bounce rate. Gmail reduces your inbox placement rate as your bounce rate rises. In extreme cases, your sending domain can be blacklisted by services like Spamhaus, which affects deliverability across all providers — not just the ones where you bounced.
Check your list before the next send
Every bounce you prevent protects your sender reputation. Start with 100 free checks — no credit card required.