When did Mal'achi prophesize?
Radak: Chagai, Zecharyah and Malachi prophesized in Bayis Sheni. For Chagai and Zecharyah, the time of their Nevu'ah is written, but not for Malachi. Perhaps he was the last of them 1 . A support is that he does not mention building Bayis Sheni, like Chagai and Zecharyah do 2 . Chazal said (Megilah 15a) that Mal'achi is Ezra 3 . We do not find that Ezra is called a Navi, only Ezra ha'Sofer 4 !
Malbim: All agree that Mal'achi prophesized after Bayis Sheni was built (i.e. also after it was built. Megilah 15a says that also he prophesized in Daryavesh's second year! - PF)
R. Yehoshua ben Korchah says so; Chachamim disagree. The Gemara favors Yehoshua's opinion. Also Targum Yonasan here says so!
Perhaps he received the name Mal'achi after he became a Navi! (PF)
What is "Masa"?
Rashi: It is a load - something handed over to Mal'achi to carry it to Bnei Yisrael.
What do we learn from "El Yisrael"?
Radak: He was sent to rebuke them for their deeds. The majority of the generation that ascended from Bavel, they persisted in their bad deeds. They married Nochriyos, like Ezra and Mal'achi rebuked them - "u'Va'al Bas El Nechar" (2:11). Also, they were not careful about Chilul Shabbos and other matters, like it says in Ezra.
What do we learn from "b'Yad Mal'achi"?
Rashi: This was handed over to him a long time ago. Chazal expounded from here (Mechilta Yisro 2, Tanchuma Yisro 11) that all the Nevi'im stood on Har Sinai, and their Nevu'os were given there. So Yeshayah said "me'Es Heyosah Sham Ani v'Atah Hashem Elokim Shelachani" (48:16).
