What a Bounce Really Tells You
When your message cannot be delivered, the receiving server returns a Non Delivery Report, often called an NDR or bounce message. Most people delete these without reading them, but the bounce contains the exact reason for failure and usually points to the fix.
Find the Error Code
Scroll through the bounce until you see a three digit code and a short sentence. Common ones are:
- 550 User unknown or mailbox unavailable: the address does not exist. Check spelling; a single wrong letter causes this. If the address is definitely correct, the recipient may have deleted that mailbox.
- 552 or Quota exceeded: the recipient mailbox is full. Call the person and ask them to clear space, then resend.
- 554 or 550 Spam detected, blocked, or listed: your message or server was treated as spam. This often relates to missing SPF or DKIM records, or sending bulk mail from a normal mailbox.
- 451 or 421 Temporary failure, try again later: greylisting or a busy server. Most systems retry automatically for up to 72 hours, so wait before acting.
- Relay access denied: your email program tried to send without authentication. Check that outgoing SMTP authentication is enabled with your full email address and password.
Step by Step Diagnosis
- Confirm the recipient address is typed correctly.
- Note whether the code starts with 4 (temporary, will retry) or 5 (permanent, needs action).
- Read the text after the code; it often names the blocklist or policy involved.
- If several different recipients bounce with spam related wording, stop sending and contact us, since your domain reputation may be affected.
When to Raise a Ticket
Open a ticket from app.techguru.co.in and paste the complete bounce message, including all headers, without editing it. The full text lets our engineers identify the blocking server and resolve listing or DNS issues quickly.
