Why do different MBPs produce different inbox results for the same campaign?

Different MBPs produce different inbox results because each uses unique algorithms, reputation databases, and user behavior models. Your domain might have excellent reputation at Gmail but limited history at Microsoft, causing the same campaign to land inbox at one and spam at the other.

User engagement patterns also vary by provider. Your Gmail subscribers might be highly engaged while your Yahoo subscribers are less active, causing the MBPs to reach different conclusions about your mail's value. Content that triggers Microsoft's SmartScreen might pass Gmail's filters cleanly.

Infrastructure differences matter too. Microsoft throttles aggressively based on IP history, while Gmail focuses more on domain signals. Yahoo weighs complaint rates heavily. These variations mean deliverability optimization requires provider specific monitoring and potentially different sending strategies for different MBP segments.