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I've just read the news that Vocus Communications have made a preliminary offer to acquire parts of TPG Telecom's core business. Whether this happens or not is likely to be announced by September 6.
That probably explains the WHY? part of their migrating their email service to TMC and the timing, but I still think it's a move whoever owns TPG Telecom will regret, even if it's not the people who initiated this rather short-sighted and stupid decision the way they are implementing it.