Esther 9:29 (ESV)

29 Then Queen Esther, the daughter of Abihail, and Mordecai the Jew gave full written authority, confirming this second letter about Purim.

Possibly, there were some Jews who doubted the validity of this new feast, as it was not prescribed in the Torah (the five books written by Moses). Whatever the case might have been, Esther thought it necessary to endorse Mordecai’s institution of the feast through a second letter, signed by both of them.

In sending this letter to the Jews, Esther and Mordecai did not lean on their royal status. This was no royal edict. Rather, they wrote as Queen Esther, daughter of Abihail and Mordecai the Jew. They were Jews writing to fellow Jews.