Hi Yutaka, To email someone (to email is the verb in this sentence). To send an email to someone (an email is the noun in this sentence). The second sentence would be gramatically incorrect without the verb.

“Email to someone” (email is not the verb). For example: “I sent an email to someone yesterday” (“sent” is the verb/action). “Email someone” (email is the verb/action). For example: “I emailed someone yesterday.” 🌈 Nicky