Did the letter begin with "Artachshasta"?
Malbim: Yes. It is the way of kings to sign their names at the head of their letters.
What is "Safar Dasa"?
Rashi: Sefer Torah.
Why does it say "Gemir"?
Ibn Ezra #1: Gemir is the Targum of "Klil Techeles" - a hint to all of Yisrael who volunteer to ascend. The prefix Lamed in "l'Ezra" applies also to Gemir and Ke'enes. He sent to Ezra, and also to Gemir and Ke'enes.
Ibn Ezra #2: It is an adjective, like Gemara Gemor; it hints to Ezra (he is learned). The Lamed in "l'Ezra" applies also to Ke'enes - refers to all nations in Ever ha'Nahar.
Malbim: This is like etc. It is a shorthand in place of listing all those to whom he sent. He says only the first (l'Ezra), Gemir and the last (u'Che'enes). This is like I explained above (4:9) "Shlam".
