Learn in the textbookHow Email Works and Specifying Recipients
DefinitionBroadcast mail (同報メール) means delivering one message with the same content to several people at once.
ExplanationMail has three address fields: To for the main people, cc for those copied for reference, and bcc for those hidden from other recipients; listing several addresses in them makes a broadcast. Who goes in which field decides what recipients can see about each other.
Memory hookRead the kanji of 同報 as "same notice" and the meaning comes back from the characters alone.