John Gill Commentary Esther 8:14

"So the posts that rode upon swift steeds that were used in the king`s service went out, being hastened and pressed on by the king`s commandment; and the decree was given out in Shushan the palace." — Esther 8:14 (ASV)

So the posts that rode upon mules and camels went out
Or on the mules, which in the Persian language were called "ahashteranim"; (See Gill on Esther 8:10),

being hastened and pressed on by the king's commandment ;
who gave them a special order to make what haste they could, that the Jews might have time to prepare for their defence, and their enemies be the more intimidated:

and the decree was given at Shushan the palace ;
the king's counsellors agreeing to it, and perhaps signing it, as they did the former; see (Esther 3:15) .