Laravel 5.6: How-to send emails, that clients will show them as threads / conversations?

My Laravel application has a ticket system included, which is sending email notifications.

All emails are built and sent like this one:

public function build()
{
    $email_from_name = "Support - " . config('app.name');
    $subject = "[" . $this->ticket->id . "] " . $this->ticket->subject . " - " . config('app.name');

    return $this->from('support@example.com', $email_from_name)
                    ->subject($subject)
                    ->markdown('emails.customer.ticket.comment_added')
                        ->with([
                            'nickname' => $this->user->nickname,
                            'ticket_id' => $this->ticket->id,
                            'ticket_subject' => $this->ticket->subject,
                            'ticket_text' => $this->ticket_comments->text,
                        ]);
}

Unfortunately, when I get multiple of these emails, no email client (Outlook, Thunderbird, Roundcube,...) shows these emails as thread / conversation. All clients show each email as "new email" thread / conversation.

What specifies, that some emails are one thread / conversation and some not? How can I tell my Laravel application, that these emails are one thread / conversation?

I thought, it just needs to be the same email subject, but it doesn't work.

via Chebli Mohamed