public class Mail {

    public Class<?> from;
    public Class<?> to;
    public Object content;


    public Mail(Object content, Class<?> to) {
        this(content, to, null);
    }


    public Mail(Object content, Class<?> to, Class<?> from) {
        this.to = to;
        this.from = from;
        this.content = content;
    }
}

Mailbox.getInstance().post(mail);

Mailbox.getInstance().atHome(this);

@OnMailReceived public void onDearMailReceived(Mail mail) {
    Toast.makeText(ConsumerActivity.this, 
                   mail.content.toString(), 
                   Toast.LENGTH_SHORT).show();
}

Mailbox.getInstance().leave(this);