Bounce reasons & SMTP error codes decoded

Sometimes when you send an email, it might bounce back (Mail Delivery Failure or similar) and you receive a “bounce message” or Non-Delivery Report (NDR) explaining why. These often contain SMTP error codes and somewhat technical text.

Private.Ki can help interpret these so you know what went wrong:

Common Bounce Reasons:

  • Invalid Address: If you mistyped the recipient’s email or it doesn’t exist, the bounce might say “User not found” or SMTP code 550 5.1.1. We can present that as: The email address you sent to does not exist. Check for typos or if the address is correct.

  • Mailbox Full: SMTP 552 or a message about quota. It means: The recipient’s mailbox is full and cannot accept new emails. You might try contacting them via another method.

  • Spam Rejection: Sometimes receiving servers reject email they think is spam. A code might be 550 5.7.1 with text about policy or spam. We could decode: Your message was rejected by the recipient’s server, possibly due to spam filters. If your email wasn’t spam, you might need to contact the recipient by other means or ask them to whitelist your address.

  • Blocked Domain/IP: Code 554 or similar: “Service unavailable” or “Blocked”. It means: The recipient’s email provider blocked the message. This could be because our server is on a blocklist or by policy. We’ll investigate if it’s our server issue, but you might inform the recipient as well.

  • Too Large: If attachment made it too big: error might say “552 message size exceeds limit.” Meaning: “The message was too large for the recipient’s email service. Try sending files through another method or compressing them.

  • Authentication Required: e.g., if misconfigured sending, but in our case as a user you wouldn’t see that because our server handles authentication to others. Might not happen to end-users.

  • DNS Issues: e.g., 450 DNS lookup failure. In clear language: Temporary failure in resolving the recipient’s domain. Usually a temporary server problem; our system will retry a few times. No action needed immediately, but if it persists, double-check the address domain.

  • Auto-Replies vs Bounces: Distinguish between a bounce and an out-of-office. Out-of-office isn’t a bounce, it’s a delivered message that’s a reply. It often has a certain format. We wouldn’t call that a bounce (the original was delivered).

  • 5.x.y Code Explanations: Possibly have a small table or knowledge doc mapping common codes (5.1.1 = Bad destination address; 5.2.2 = mailbox full; 5.1.0 or 5.4.x = domain issues, etc.). In support center, we likely have an article decoding these codes.

  • Time-of-bounce vs deferral: Some errors are persistent (hard bounce) vs temporary (soft bounce). Temporary ones (4xx codes like 450, 421) our servers will usually reattempt automatically over some hours. So the user might not see a bounce immediately. They’d get a delayed bounce if after trying, it still fails. If they get a “This is a warning: message not yet delivered for 4 hours,” that’s a delay notification. We should decode that as “Delivery is delayed – typically a temporary server issue, we will keep trying. No action needed yet, this is just an alert.” If final failure, then bounce.