What is a bounce in email?
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You hit send, and instead of landing in someone's inbox, your email comes right back to you with an error. That's a bounce. It means the receiving mail server couldn't (or wouldn't) accept your message, and you got a notice back saying why. That is the everyday meaning, anything that comes back undelivered, and it is the one we use. Strictly speaking, a refusal during the sending conversation is called a rejection, and a bounce is mail the server accepted first and sent back later. What's the difference between "rejected" and "bounced"? explains the two. For your list they mean the same thing. And not every failed email bounces. Some are accepted and then quietly dropped or held in quarantine, some go to spam, and a catch-all domain accepts mail for addresses that do not exist. None of those sends anything back, so no bounce does not mean it arrived.
Bounces fall into two main categories. Hard bounces are permanent failures. The address doesn't exist, the domain is gone, or the server is flat-out refusing delivery for that recipient. You shouldn't send to those addresses again. Ever. Soft bounces are temporary failures. The mailbox is full, the receiving server had a blip, or your message hit a temporary 4xx size limit. The address might work fine tomorrow.
Every bounce comes with an SMTP error code that tells you what went wrong. A 550 code with "user unknown" is a clear hard bounce. A 451 is a temporary error. It can mean a local processing problem on the receiving server, and it is the server asking you to try again later. Reading those codes is how you figure out whether to suppress an address or just wait it out.
Why does this matter for your deliverability? Mailbox providers like Gmail and Outlook watch your bounce rate closely. A high bounce rate signals that your list is messy, and a messy list is a red flag for poor sending practices. Keep sending to bounced addresses and your sender reputation will take a hit that affects your entire sending domain, not just the messages going to bad addresses.
Most ESPs handle bounce suppression automatically. Hard bounces get removed right away. Soft bounces get retried a few times before they're suppressed. It's worth checking your platform's bounce policy so you're not accidentally relying on defaults that are too lenient.
If your bounce rate feels higher than it should be, your list may need a clean. You can run it through Review My Emails and we'll flag what's safe to send to and what to suppress. Your first 1,000 addresses are free. Or if you're seeing specific error codes you can't interpret, drop them in our SOS hotline and we'll help you decode them.
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