我会这样做：

class ContactanosMailable extends Mailable
{
    use Queueable, SerializesModels;

    public $siniestro;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct($siniestro)
    {
        $this->siniestro = $siniestro;

        // Set the subject property
        $this->subject = $siniestro->siniestro;

        // Or use the subject method
        $this->subject($siniestro->siniestro);
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->view('emails.contactanos');
    }
}

使用subject（）方法看起来更简洁，但两者都有效。

public function build(){
     
        $from_email = "test@mail.com";
        $subject = "Account Request";
        return $this->from($from_email)->subject($subject)->view('emails.contactanos')
            ;
    }