It is clear from the referenced text that D'Artagnan deceived Milady (Lady de Winter) by using Kitty to hack the exchange of correspondences between Milady and the Comte. This way, he is able to send her a fake letter using the Comte's name, requesting that to meet at her house.
Milday in the above named story is a Cardinalist agent and the villain of the story.
Thus, by hacking the correspondence between Midaly and Compte, D'Artagnan deceived Milady.
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