Email troubleshooting guides
Zoho delivery errors, organized by problem
Troubleshoot Zoho delivery failures using the exact SMTP code and response wording.
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Zoho 421 4.7.0: Zoho temporary policy deferralZoho 421 4.7.0: Zoho temporary policy deferral. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 451 4.4.1: Zoho destination did not answerZoho 451 4.4.1: Zoho destination did not answer. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 530 5.7.0: Zoho authentication requiredZoho 530 5.7.0: Zoho authentication required. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 550 5.1.1: Zoho recipient does not existZoho 550 5.1.1: Zoho recipient does not exist. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 550 5.1.2: Zoho destination domain invalidZoho 550 5.1.2: Zoho destination domain invalid. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 550 5.7.1: Zoho delivery not authorizedZoho 550 5.7.1: Zoho delivery not authorized. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 550 5.7.23: Zoho SPF validation failedZoho 550 5.7.23: Zoho SPF validation failed. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 550 5.7.30: Zoho DKIM validation failedZoho 550 5.7.30: Zoho DKIM validation failed. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 552 5.2.2: Zoho recipient mailbox fullZoho 552 5.2.2: Zoho recipient mailbox full. Understand the response, identify ownership, and follow an evidence-based next step.
Zoho 554 5.6.0: Zoho malformed messageZoho 554 5.6.0: Zoho malformed message. Understand the response, identify ownership, and follow an evidence-based next step.
Primary sources: Primary technical reference.