Mordecai knew the decree before ordinary news could reach him.
Targum Sheni on (Esther 4:1) says he saw everything by Ruach HaKodesh, the holy spirit of prophetic insight. The king's servants had already carried messages through the empire. Scribes from one hundred and twenty-seven provinces sat at the gates of Shushan with rolls and books, writing decrees against the Jews and their laws.
The targum expands the royal letter into a full imperial accusation. Haman is presented as a useful outsider who helps the king prevail against supposed enemies. The Jews are described as disobedient and dangerous. Violence is dressed in bureaucratic language.
Mordecai sees through it. He does not need the empire's explanation of itself. Prophetic insight shows him the spiritual shape of the danger: a sealed decree, written in the king's name, aimed at the whole people. That is why his mourning is immediate. The crisis is not rumor. It is already a heavenly and political fact.
And Mordecai, the righteous, saw by the Holy Spirit everything that had been done, viz. that the king had sent word from his palace by his servants to the righteous Haggai, Zechariah, and Malachai, who were in the chamber of hewn stones, and were prophesying on the great wall of Jerusalem, that they should stop the work of building after seventy-two towers were already built. The wicked Ahhashverosh also fetched a hundred and twenty-seven scribes from a hundred and twenty-seven provinces, every one of whom had a roll and a book in his hand. They sat at the gates of Shushan and wrote, and sent out grievous decrees concerning the Jews and their laws. The first letter written in the name of the king, and sealed with his signet-ring, they despatched by swift messengers, and the contents were as follows:—"From me. King Ahhashverosh, to all peoples, nations, and languages who live in all the land, peace be multiplied. I make known to you, that a certain man came to us, who is not from our place nor from our province, and he came for the purpose of joining us, that we might prevail against our enemies. We have made investigation concerning him, and (we find) that his name is Haman, son of King Agag, son of the great Amalek, son of Reuel, son of Eliphaz the first-born of Esau, in fact, a descendant of prominent lords and wealthy people. This man asked of me a small and insignificant petition, and informed me concerning the Jews and their blameworthy laws and affairs. He said, 'When they came out from Egypt they numbered six hundred thousand men, and so I will give thee six hundred thousand minas of silver, a mina for every man' for which sum he desired that I should sell this people to him to be killed.
