(to send): Sie schickt ihrem Freund (Dat) eine E-Mail (Akk).
In English, verbs rarely dictate the case of an object. We say “I help you” regardless of who is doing what. In German, however, the verb is the king of the sentence. Every verb has a specific valence—a set of cases it requires. List Of Accusative And Dative Verbs In German Pdf
(to write): Er schreibt seiner Mutter (Dat) einen Brief (Akk). (to send): Sie schickt ihrem Freund (Dat) eine E-Mail (Akk)