title: Laravel 5.6: How-to send emails, that clients will show them as threads / conversations?
body: <p>My Laravel application has a ticket system included, which is sending email notifications.</p>
  
  <p>All emails are built and sent like this one:</p>
  
  <pre><code>public function build()
  {
      $email_from_name = "Support - " . config('app.name');
      $subject = "[" . $this-&gt;ticket-&gt;id . "] " . $this-&gt;ticket-&gt;subject . " - " . config('app.name');
  
      return $this-&gt;from('support@example.com', $email_from_name)
                      -&gt;subject($subject)
                      -&gt;markdown('emails.customer.ticket.comment_added')
                          -&gt;with([
                              'nickname' =&gt; $this-&gt;user-&gt;nickname,
                              'ticket_id' =&gt; $this-&gt;ticket-&gt;id,
                              'ticket_subject' =&gt; $this-&gt;ticket-&gt;subject,
                              'ticket_text' =&gt; $this-&gt;ticket_comments-&gt;text,
                          ]);
  }
  </code></pre>
  
  <p>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.</p>
  
  <p>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?</p>
  
  <p>I thought, it just needs to be the same email subject, but it doesn't work.</p>