The Vilna Gaon points out that Mordechai provided these animals to the couriers because he wanted them to hurry. This, despite the fact that they were exhausted from having just traversed the largest nation in the world to deliver Haman’s original decree. Seeing that they were tired, he gave them the fastest possible horses.
The Malbim writes that Mordechai sent the messengers on horses in contrast with Haman. In explanation, R’ Chaim Kanievsky writes that Haman had plenty of time – he had eleven months. Mordechai is in a hurry to save lives.
Interestingly, the Talmud (Megillah 18a) writes that the sages were unsure as to the translation of the couriers’ transportation.
Rashi translates achashtirans as swift camels.
The Ibn Ezra writes that these are a species of mule. After all, the verse says they are bred from ramachs, and the Mishnah (Kilayim 8:5) considers a ramach a mare, mother of a mule. Also, the Arabic word, ramach means mare.
R’ Yosef Kimchi concurs and he adds that achash in Median means large and tiran (misrain) means two. Therefore, the combination of the two words means the mating of two large animals: the horse and the donkey.
R’ Samson Raphael Hirsch (Collected Writings, Volume IV, 286) translates rachash as a draft horse. Parenthetically, he adds that the symbolic meaning of these in TaNaCh indicates a reluctance to listening to one’s master.
R’ Yehoshua Leib Diskin writes that these untranslatable words answer another question from the Talmud (Megillah 3b), which says one must interrupt Torah learning to hear the public reading of Megillas Esther on Purim. This is also brought down as the Halacha (Shulchan Aruch Orach Chaim 687:2). The Halacha (Mishnah Berurah Orach Chaim 690:26) further states from this verse that one fulfills one’s obligation in Hebrew despite not knowing the meaning. But is not Megillas Esther also Torah?! Rav Diskin explains that it is not considered Torah study if one does not understand it. Understanding is an essential component of Torah study. Hearing the reading is still an obligation because persumei nisa (publicizing a miracle) is even greater than Torah study.
Aside for Mordechai’s desire to send these letters in Sivan for the reasons mentioned above, the Yosef Lekach writes that Mordechai waited for Haman’s couriers to return from their original mission (Esther 3:13). Utilizing the same couriers would add legitimacy to Mordechai’s letter.
The Midrash (Bireishis Rabba 100:6) writes that H-Shem rewarded the gentiles for mourning Yaakov for 70 days (Bireishis 50:3) by giving them these 70 days between the 13th of Nisan and the 23rd of Sivan to do teshuva.
R’ Yehonason Eibshutz writes that a common calculation in the Torah is a day for a year, as when the Jewish people were punished (Bamidbar 14:17) with 40 years of delaying their entry into the Holy Land for their believing the spies who traversed the land for 40 days. Therefore, he writes, these 70 days were for the Jews to perform teshuva in gratitude for H-Shem’s saving their lives, which the verse (Tehillim 90:10) says lasts an average of 70 years.
Similarly, the Vilna Gaon explains that the Jews were scared about their fate for these 70 days to get an atonement for the 70 years of exile which they had caused upon themselves.
R’ Dovid Feinstein writes that these 70 days represented the 70 nations of the world were allowed to think that they were in control of the fate of the Jews.