Why do emails bounce?
An email bounces when the receiving mail server refuses it or cannot deliver it and returns a failure notice. Hard bounces are permanent: the mailbox or domain does not exist, or the server rejects the sender by policy. Soft bounces are temporary: a full mailbox, a server outage, an oversized message or rate limiting. The SMTP code in the bounce message tells you which.
| Cause | Type | Typical code | What to do |
|---|---|---|---|
| Mailbox does not exist (typo, deleted, person left) | Hard | 550 5.1.1 | Remove the address; find a current one |
| Domain does not exist or has no MX record | Hard | 5.1.2 or DNS failure | Remove; check for domain typos |
| Rejected by policy, blocklist or failed authentication | Hard or soft | 5.7.x | Fix SPF, DKIM and DMARC; check reputation |
| Mailbox full | Soft | 4.2.2 or 5.2.2 | Retry later; suppress if it repeats |
| Server busy, offline or throttling you | Soft | 421, 450, 451 | Retry with backoff; slow your send rate |
| Message too large | Soft or hard | 552 5.3.4 | Shrink attachments; send a link instead |
What a bounce actually is
When you send an email, your mail server opens a conversation with the recipient's server using SMTP. At each step the receiving server answers with a status code. If it accepts the message, delivery is complete from the sender's side. If it refuses, or accepts and then fails to deliver, a non-delivery report (often called a bounce message) comes back to you.
The code tells you what kind of failure it was:
- 5xx codes are permanent failures. Sending again will not help. These are hard bounces.
- 4xx codes are temporary failures. The sending server usually retries for a while before giving up. These are soft bounces.
Most bounce messages also include an enhanced status code such as 5.1.1, which narrows the cause down. An out-of-office or auto-reply is not a bounce: the message was delivered, and the reply is a separate email.
Common causes of hard bounces
The mailbox does not exist. This is the most common cause. The address may contain a typo, the account may have been deleted, or the address was guessed from a pattern (first.last@company.com) and never existed. In B2B lists, a frequent reason is that the person left the company and their mailbox was shut down.
The domain does not exist or cannot receive mail. A misspelled domain (northwind.cm instead of northwind.co), an expired domain or a domain with no MX record will fail before any mailbox is checked.
The receiving server blocks the sender. Policy rejections usually carry a 5.7.x code. Causes include the sending IP or domain appearing on a blocklist, poor sender reputation, content flagged as spam, or failed authentication. Major mailbox providers increasingly reject mail that lacks valid SPF, DKIM and DMARC records, so a misconfigured domain can bounce mail to perfectly good addresses.
Common causes of soft bounces
The mailbox is full. The account exists but has reached its storage quota. Some servers report this as temporary, others as permanent.
The server is unavailable or overloaded. Maintenance, outages and connection timeouts produce 4xx responses. Your server will normally retry automatically.
You are being rate limited. Sending many messages to one provider in a short window can trigger throttling, often with a 421 or 451 code. New domains and new sending IPs hit this more often.
The message is too large. Big attachments or heavy inline images can exceed the recipient's size limit.
Greylisting. Some servers deliberately reject the first attempt from an unfamiliar sender with a temporary code and accept a retry a few minutes later. This usually resolves on its own.
A soft bounce that repeats on every send for the same address should be treated like a hard bounce and suppressed.
Why prospecting lists bounce more than newsletters
People who sign up for a newsletter typed their own address and confirmed it. Cold outreach lists have no such check, so several extra risks stack up:
- Data decay. People change jobs, companies rebrand and domains get retired. A list that was accurate a year ago may not be today.
- Guessed addresses. Pattern-based guesses look plausible but have never been confirmed.
- Catch-all domains. A catch-all domain accepts every address during the SMTP conversation, so no one can confirm a specific mailbox. Some of these servers then bounce the message later, or drop it silently.
- Role addresses and disposable addresses. Shared inboxes like info@ and throwaway domains are more likely to fail or to generate complaints.
High bounce rates damage sender reputation, which then causes policy rejections on good addresses too. See what is a good email bounce rate for typical thresholds.
How to prevent bounces before you send
- Authenticate your sending domain with SPF, DKIM and DMARC, and warm up new domains gradually. More on this in how to improve email deliverability.
- Verify every address before the first send. Verification checks syntax, DNS and MX records, and runs a mailbox-level check. It catches nonexistent mailboxes and dead domains before they cost you reputation.
- Treat uncertain results as uncertain. Catch-all and unknown addresses are not the same as verified. Send to them at lower volume, or hold them back, rather than mixing them in with confirmed addresses.
- Suppress hard bounces immediately and never resend to them.
- Re-verify lists that are more than a few weeks old, especially before a large campaign.
Verification lowers bounce rates but cannot remove them entirely. A mailbox can be deleted between the check and the send, and catch-all servers can accept and then reject.
Example. Maya Chen, VP Revenue Operations at Northwind Analytics, is about to launch a campaign to a fictional list of 400 contacts exported from an old CRM. Before loading it into her sequencer, she runs the list through an email verifier. Most addresses come back verified, a group sits on catch-all domains, a few return unknown, and some are invalid because the people have left their companies. She removes the invalid ones, sends to the verified group at her normal volume, and puts catch-all and unknown addresses in a separate, slower campaign. For the contacts who moved, she looks up their new role and current work email.
If you are working with an AI agent, Leadsets offers this check as the verify_email tool over MCP or REST. It labels each address verified, catch-all, unknown or invalid with a confidence score, and does not report uncertain results as valid. Details are on the email verification page.
Questions and answers
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent failure, such as a mailbox or domain that does not exist, and is signalled by a 5xx SMTP code. A soft bounce is a temporary failure, such as a full mailbox or a busy server, and is signalled by a 4xx code. Hard bounces should be removed from your list right away, while soft bounces can be retried.
Why did my email bounce if the address is spelled correctly?
A correctly spelled address can still bounce if the mailbox was deleted, the person left the company, or the receiving server rejected your message by policy. Policy rejections often come from failed SPF, DKIM or DMARC checks, a blocklisted sending IP or domain, or poor sender reputation. Read the code in the bounce message: a 5.7.x code points to a sender problem rather than a bad address.
Does a bounced email mean the person left the company?
Often, but not always. A 5.1.1 'user unknown' bounce at a company domain commonly means the account was closed, which frequently happens when someone leaves. It can also mean the address was mistyped or guessed incorrectly in the first place.
Should I resend an email after a soft bounce?
Most sending servers retry soft bounces automatically for a period before giving up. If the same address soft bounces on several separate campaigns, treat it as undeliverable and suppress it. Repeatedly sending to failing addresses can hurt your sender reputation.
Can email verification prevent all bounces?
No. Verification removes most nonexistent mailboxes and dead domains before you send, but a mailbox can be deleted after it was checked. Catch-all domains cannot be confirmed at the mailbox level, so some of them will still bounce.
Do bounces hurt my sender reputation?
Yes. Mailbox providers watch how often a sender hits nonexistent addresses, and a high hard bounce rate looks like a poorly maintained or purchased list. That can lead to more filtering and policy rejections, even for valid recipients.
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