Learn Italian: Ogni richiesta è stata respinta da parte dell'amministrazione. - Every request was rejected by the administration.

While both mean 'by the administration', using da parte di is very common when the agent performing the action is an institution, collective group, or organization. It emphasizes the institutional source of the decision and gives the sentence a slightly more formal or bureaucratic tone.

Why do we need stata in è stata respinta?

This forms the passive voice in the passato prossimo tense. The active equivalent is ha respinto (has rejected). To make it passive, we use the auxiliary essere in the passato prossimo (è stata) plus the past participle of the main verb (respinta). Notice that both stata and respinta end in '-a' to agree with the feminine singular subject, Ogni richiesta.

Why is the noun singular (richiesta) when the sentence describes multiple rejected requests?

The adjective ogni means 'every' or 'each'. Just like in English, it is invariable and must always be followed by a singular noun. If you wanted to use a plural noun to express the same idea, you would have to change the phrasing to Tutte le richieste (All the requests).

Could I use rifiutata instead of respinta here?