Why do emails bounce?
Updated September 3, 2026
A bounce is the receiving server refusing your message. Whether that refusal is permanent (hard) or temporary (soft) decides how much damage it does to your sender reputation.
Hard bounces: permanent failures
The mailbox does not exist, the domain does not exist, the domain has no mail server, or the server has explicitly declared it accepts no mail. Hard bounces are the expensive kind: inbox providers read a high hard-bounce rate as proof that you do not know who is on your list.
- Invalid or misspelled domain (gmial.com, hotmial.com).
- Deactivated mailbox after someone leaves a company.
- Domain expired or parked with no MX record.
- Null MX record — the domain refuses all mail by design.
Soft bounces: temporary failures
The mailbox is full, the server is down or throttling you, the message is too large, or greylisting is delaying you. Soft bounces usually retry automatically, but repeated soft bounces to the same address should be treated as a hard bounce after a few attempts.
Bounces caused by you, not the recipient
Missing or misaligned SPF, DKIM and DMARC records, a cold sending domain, a blacklisted IP, or spam-triggering content can all produce refusals that have nothing to do with the address being wrong. Check your authentication records before blaming the list.
What a healthy bounce rate looks like
Under 2% is normal; over 5% is where providers begin throttling and filtering. If you are above that, stop sending and clean the list before the next campaign.
How to prevent bounces before you send
Run the free bounce-risk checks — syntax, TLD, DNS/MX, parked domains, disposables, role accounts — then SMTP-verify the survivors. Most hard bounces are removable before a single message leaves your server.
Try it on your own list
Run the free validator on any address or CSV, then spend credits only on the addresses that survive.
Frequently asked questions
Keep reading
- How to verify an email address
Step-by-step guide to verifying an email address for free: syntax, domain, MX records, disposable checks and SMTP verification, plus how to check a whole list.
- Email validation vs email verification
Validation checks whether an address could receive mail; verification confirms the mailbox exists. Learn what each one catches, what it costs and when to use both.
- How to clean an email list
A practical checklist for cleaning an email list: dedupe, fix typos, drop dead domains, remove disposables and role accounts, then verify the survivors before you send.