<?php

namespace Webup\LaravelHelium\Contact\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;

class ContactMessage extends Mailable
{
    use Queueable, SerializesModels;

    private $contact;

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

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->subject('[Site] contact')
            ->text('helium::emails.contact-message')
            ->with(['contact' => $this->contact]);
    }
}

<?php

namespace Webup\LaravelHelium\Contact\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;

class ContactMessage extends Mailable
{
    use Queueable, SerializesModels;

    private $contact;

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

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->subject('['.config('app.name').'] Contact')
            ->text('helium::emails.contact-message')
            ->with(['contact' => $this->contact]);
    }
}

Update subject value (Use app.name)
Update subject value (Use app.name)