Emails bouncing

Updated 9 October 2026

Short answer

A bounce is the receiving mail server refusing your email and sending back a reason. Read that reason. If it's about the address ("user unknown", "mailbox unavailable"), the address is wrong or closed: remove it. If it's about you ("authentication", "policy", "blocked", "reputation"), your domain's SPF, DKIM or DMARC setup or your sending reputation is the problem, and it will affect many recipients.

What's the problem

Emails come back as "Undeliverable" or "Delivery Status Notification (Failure)", or your email service shows bounces.

Why it happens

  • A misspelled or non-existent address.
  • A full or disabled mailbox.
  • Missing or failing authentication: SPF, DKIM or DMARC not set up or not matching your From address.
  • Poor sending reputation: previous spam complaints or lots of bounces.
  • The recipient's server blocked you by policy.
  • Repeat sends to addresses that already bounced, which email services often suppress.

How to fix it

  1. Read the bounce message and note the code and reason.
  2. For address problems, correct or remove the address and stop sending to it.
  3. For authentication problems, set up SPF, DKIM and DMARC for your sending domain, matching the domain in your From address.
  4. Clean your list: remove addresses that bounce and people who don't engage.
  5. Check your email service's suppression list if a corrected address still doesn't receive mail.

What Preventionlabs checks

One click runs the full report on your domain. For this problem it shows your email authentication: SPF (and its 10-lookup limit), DMARC and DKIM; whether mail to you must arrive encrypted (MTA-STS) and whether you get reports when it does not; your DNS: nameservers, addresses, DNSSEC, and records pointing at services that no longer exist. It can't see your sending provider's delivery logs, so check bounces there.

Run your free report

Your first report is free, no card needed. Then A$50 a month, up to 10 domains. Read-only: it never changes anything.

Sources

  1. Twilio SendGrid: Manage bounced messagesofficial docsCommon causes for bounced email messages include a misspelled or nonexistent email address .
  2. Google: Email sender guidelineshelp centerSet up DMARC email authentication for your sending domain.

Related

SMTP error SMTP errors start with a code: 4xx means temporary (try again later), 5xx means permanent (fix something first). The text after the code says what. Emails not sending from my website Check whether the email left your site at all, then whether it was rejected or landed in spam. Most failures are missing SPF/DKIM or wrong sending settings. SendGrid error Check SendGrid's activity feed for the message, then the error. Common causes: an unverified sender identity, a wrong or revoked API key, or bounces. Mailgun error Most Mailgun problems come from an unverified domain (a daily limit and "sent via" labels), a wrong API key, or sandbox domain sending rules.
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