Understand a bounced email
A bounce tells you exactly what went wrong, in a format designed for machines. This turns it into a sentence, and tells you whether to fix something or delete an address.
- Runs entirely in your browser
- Works offline
- No upload
- No sign-up
- No watermark
Loading the tool…
How it works
- Paste the whole bounce message, headers and all.
- Read the verdict: permanent means stop sending; temporary means the server will keep trying.
- Read the diagnostic and the signals to find the actual cause.
Why nothing is uploaded
Every operation on this page is done by code running inside your browser tab, using the same engine that renders web pages. The file is read from disk into your tab’s memory, transformed there, and written back out as a download. It is never sent anywhere — not to us, not to a third party.
Verify it yourself
- Open your browser’s developer tools (F12) and select the Network tab.
- Load your file and run the tool.
- The only requests you will see fetch the tool’s own code — and, for a few heavy tools, their open-source engine from a public CDN — plus one small page-view ping to loreatec.jp (page address and title, nothing more). None of them carry your file.
Frequently asked questions
Hard bounce or soft bounce — why does it matter?
A 5.x.x code is permanent: the address is wrong, the mailbox is gone, or you have been rejected on policy. Sending again does not help and, when you keep doing it, the receiving side reads it as a sign of a list you did not clean — which damages the reputation of everything you send. A 4.x.x is temporary and the sending server retries on its own, typically for several days.
What does 5.7.26 from Gmail mean?
Authentication. Since 2024 Gmail requires bulk senders to pass SPF and DKIM and to align at least one of them with the domain in the visible From address. A message that fails is rejected outright rather than filtered. Set up DKIM signing on the From domain and publish SPF and DMARC for it.
The bounce mentions ドメイン指定受信. What can I do?
That is a filter the recipient set on their own phone, allowing mail only from domains they listed. Nothing you configure on your side bypasses it — the recipient has to add your domain. What you can do is make sure you pass SPF and DKIM, because the separate なりすまし規制 filter rejects mail that fails authentication regardless of any allow-list.
There is no status code anywhere in my bounce.
Some servers send a plain-language message with no structured part at all. The tool then looks for an SMTP code and known phrases in the text. If it finds nothing, read the diagnostic quoted from the receiving server — that line is written by whoever rejected the message and is usually the most specific thing available.