| Bounce Code | Message | Reason |
|---|
| 500 5.5.2 | unrecognized command | Wrong SMTP command sent by the client application. |
| 501 5.5.2 | MAIL FROM syntax error | FROM email address is in the wrong format. |
| 501 5.5.2 | RCPT TO syntax error | TO email address is in the wrong format. |
| 503 5.5.1 | RCPT TO without MAIL FROM | FROM email address is missing. |
| 503 5.5.1 | DATA without MAIL FROM | Client tries to send data without prior sending mail from |
| 503 5.5.1 | DATA without RCPT TO | TO email address is missing |
| 554 | Transaction failed[Missing Header X-ApiKey] | The X-ApiKey is missing from the email headers. |
| 554 | Transaction failed[Missing Header X-CampaignID] | The X-CampaignID header is missing. (Required ONLY if you log in to app.maropost.com) |
| 554 | Transaction failed[Invalid X-CampaignID] | The X-CampaignID header is in the wrong format. The Campaign ID is always an integer. |
| 554 | Transaction failed[Missing Required Headers From or To[see RFC2822 section 3.6]] | The message sent to Maropost’s SMTP API is missing the standard required headers |
| 554 | Transaction failed[Missing Header X-CampaignName] | X-CampaignName is missing from the email header |
| 554 | Transaction failed[Missing Header X-AccountID] | X-AccountID is missing from the email header |
| 554 | Transaction failed[Invalid Header X-AccountID] | X-AccountID is in the wrong format. The Account ID is always an integer. |
| 554 | Transaction failed[Service Temporarily Unavailable. Please try after a few minutes.] | An extremely rare response that may occur if the SMTP service is temporarily unavailable. |