The problem is that there's no proof that the email - or any of the previous emails and letters - is genuine. It/they may be but it cannot be proved, unless perhaps by each of us contacting the Company. And that is unrealistic. An RNS, however, can be accepted without question as having come from the Company.
As I've posted before, the Company seem to make a point of issuing an RNS in time for the agm. This can hardly be by accident, as it's happened every year. So, we can assume that there will be one later in the month. It's exact timing is completely up to the Company. Whether its contents refer to the milestone payments or something else is also the Company's decision. The main point here is that it's very unlikely to be bad.