SMTP & NDR Code Decoder

Decode SMTP reply + enhanced status codes and Exchange NDR codes.

Why this exists

Bounce-backs are the highest-volume mail ticket and the codes are cryptic, 5.7.1, 5.4.1, 4.4.7. Decoding the reply code and enhanced status into 'recipient rejected you for policy reasons' points straight at the fix.

How to use

Paste a whole bounce or NDR, or just a code like 550 5.7.1 or 5.7.1.
It pulls out the reply code and the enhanced status code and decodes both: class (permanent/transient), subject, and detail in plain English.
Common Exchange Online and on-prem causes are listed where known.

Paste a bounce / NDR (or a single code)

No need to isolate the code, paste the full message. You can also search by keyword (e.g. relay, spam, mailbox full) across all 94 codes.

Drop in a non-delivery report and both codes are decoded into a class (permanent / transient), a plain-English subject and detail, plus the likely Exchange Online cause. Try one: