After looking at the overall direction of email the next question is does this vary by rank? The graph below shows the direction of email by rank; as might be expected rank 0 (the most senior) can only send to lower ranks (there is only 1 rank 0) and rank 8 cannot send downwards. In-between the shape of the curve is remarkably well behaved; I would say this does not show much bias at any rank, considering their position:

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