Mailgun error

Updated 9 October 2026

Short answer

Start by checking your sending domain is verified in Mailgun. Mailgun's docs say verified domains aren't held to a 300-emails-a-day limit, don't show "sent via Mailgun.org", and are less likely to be disabled. Then check the API key. If you're testing with Mailgun's sandbox domain, it can only send to authorised recipients.

What's the problem

Your app gets an error from Mailgun, emails stop after a number per day, or recipients see "sent via mailgun.org".

Why it happens

  • The domain isn't verified: DNS records Mailgun asked for are missing or wrong.
  • The API key is wrong or revoked.
  • Sandbox domain restrictions on who you can send to.
  • Bounces or complaints affecting your account.

How to fix it

  1. Check your domain's status in Mailgun and add any DNS records still showing as unverified.
  2. Confirm the records are live with a public DNS lookup.
  3. Check the API key in your live environment.
  4. Move from the sandbox to your own verified domain for real sending.
  5. Check Mailgun's logs for each failed message's reason.

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. Mailgun: Domain Verificationofficial docsVerified domains are not subject to a sending limit of 300 emails per day
  2. Mailgun: Domain Verificationofficial docsMailgun is less suspicious of traffic that is being sent on verified domains and that reduces the likelihood of being disabled
  3. Mailgun: Sandbox Domainofficial docsSandbox domains can only send to authorized recipients.
  4. Mailgun: Domain Verificationofficial docsNo more "sent via Mailgun.org" message in your emails

Related

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. 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 bouncing A bounce means the receiving server rejected the email. Read the bounce message: a bad address, a full mailbox, or an authentication or reputation problem. 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.

Preventionlabs is independent and not affiliated with Mailgun. Product names are used only to describe the problem.

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