The letter appears to be genuine. The firm is genuine and the person named at the top of the letter does work for the firm although I can’t find her listed on the Law Society website, there is press comment about her work which does include this type of work.
I can’t find anything which suggests that Microsoft use this firm but certainly, the content of the letter is credible. It may not be accurate but it is correctly laid out.
It would appear, that in view have not carried out any infringing activities, someone has reported this or someone from Microsoft has picked it up without being aware of the history. What you mustn’t do is ignore this.
I suggest that you write back to the firm saying that you have received the letter and you note the contents and that you can reply specifically to section 2 and section 3.you deny any wrongdoing because all the items that you sold have been genuine.
Say that there have been spurious complaints andallegations of unlicensed transactions previously made to their client and that this was investigated by their client on the date that it was investigated and give them the name of whoever it was that you dealt with. Preferably, send also a copy of the correspondence and suggest that they contact their client directly.
That should hopefully be the end of the matter.
Can I clarify anything for you?
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We can still exchange emails. Best wishes.