What is a bounce in email?

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 it sent back a notification explaining why.

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 was too large. 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 "try again later" is the server asking you to be patient. 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 RME Clean and we'll flag what's safe to send to and what to suppress. 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.