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Glossary

Hard bounce

A hard bounce is a permanent delivery failure, such as an address that does not exist, which the receiving server reports with a 5xx status code.

Updated · By Otman Ounana · How we research

Why it matters when you send from your own Gmail

Each hard bounce tells mailbox providers that you are mailing addresses you have not checked. Google advises that if messages start bouncing, you reduce the sending volume until the error rate falls.[1] The address should never be tried again. Honest Intros stops the sequence and suppresses the address when a hard bounce arrives, and pauses new first emails from a mailbox after three bounces in its last thirty first emails.

Example

You write to jane@oldcompany.com, a mailbox that was closed last year. Within seconds a delivery failure arrives saying the user is unknown. That is a hard bounce.

What to do

Remove a bounced address at once and look for a pattern. Several bounces at one company often mean a changed address format or a closed office, and the rest of that list deserves a second look.

Related terms

All terms are on the glossary.

Read next

  • Can cold email get Gmail suspended?When Google pauses or suspends sending from a Gmail or Workspace account, for how long, and the habits that keep a mailbox used for cold email safe. Quoted from Google.
  • Warm up the inbox you already useWarming up an existing Gmail or Workspace mailbox for cold email: what Google advises, a day-by-day schedule from 20 to 50 a day, and when to slow down.
  • List-UnsubscribeList-Unsubscribe is an email header that carries an unsubscribe address, so a mail app can show its own Unsubscribe button without the recipient opening the message.
  • Spam trapA spam trap is an email address that real people do not use, kept by a mailbox provider or a blocklist operator to catch senders who mail addresses they should not.

Sources

  1. Google, Email sender guidelines. “If messages start bouncing or start being deferred, reduce the sending volume until the SMTP error rate decreases.” Read on 1 October 2026.