@Entity
@Table(name = "Users")
public class Users implements  Serializable {

    private static final long serialVersionUID = -5638228567536191397L;

    @Id
    @Column(name = "_id")
    private ObjectId _id;
    private String name;

    @ElementCollection
    private List<String> mail;

//getter, setter ...
}

TypedQuery<Users> query = em.createQuery("SELECT u FROM  Users u JOIN u.mail m WHERE m ='abc@blah.com' ", Users.class)
List<Users> users = query.getResultList();