sysop@hub:~/tools/mailflow-debugger$

mailflow-debugger

"My mail came back." Paste the bounce message here, or the error line from a scanner, a message trace result, or the spam headers of a received mail. You get the cause in plain language, which side has to fix it, the commands to check it and a reply you can send to the user. The text is read by your browser and never uploaded.

or just a code, for example 5.7.509

Try an example:

what you can paste

Input	Where to get it
Bounce message (NDR)	The mail the sender got back. Copy all of it, including the part headed "Diagnostic information for administrators". That part holds the status code and the name of the rejecting server.
SMTP error line	From the display or log of a scanner, printer, application or mail server, for example `550 5.7.1 Unable to relay`.
Message trace	The output of `Get-MessageTraceV2` or `Get-MessageTraceDetailV2`, or a row copied from the trace in the Exchange admin center.
Spam headers	The `X-Forefront-Antispam-Report` and `Authentication-Results` lines from the source of a mail that landed in Junk or quarantine.
Just a code	An enhanced status code such as `5.4.1` on its own. The diagnosis is less specific without the text around it.

how to read a status code

Part	Meaning
First digit	`2` delivered, `4` temporary failure (the sending server tries again), `5` permanent failure (the mail is returned).
Second digit	`1` address, `2` mailbox, `3` mail system, `4` network and routing, `5` protocol, `6` content, `7` security and policy.
Third part	The detail. Microsoft, Google and others define their own numbers here, which is why `5.7.509` or `5.7.26` only make sense for one provider.
Three-digit number in front	The classic SMTP reply, such as `550` or `451`. It says less than the code that follows it.

The debugger knows 50 situations, with a focus on Exchange Online and the big mail providers. It works from the text you paste and does not look anything up. A bounce is written by the server that rejected the mail, so the wording differs between products, and a rare or vendor-specific message may only get the general reading of its code. When in doubt, the administrator of the rejecting server has the final answer in their logs.