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|---|---|---|---|
Inactive mailbox | 550 5.1.1, "Mailbox not found", "Disabled mailbox" | Damages sender reputation, increases bounce rate. | Immediately remove from list. |
Temporary issue | 4xx transient errors (e.g., "temporary delivery failure") | Minor, may cause delays, but often resolved by retries. | No action needed if resolved, monitor patterns. |
Content/Reputation issue | 5xx permanent errors, often vague descriptions. | High impact, likely indicates spam filtering or blocklisting. | Review content, authentication, and sender reputation. |
