title: MailKit : search if custom header exists
body: <p>I have added a custom header in emails, conditionally</p>
  
  <p>and I want using the IMAP/SearchQuery to return only  the mails having the   header.</p>
  
  <p>Not its value, only to return true/false if the header exists.</p>
  
  <p>Is it possible ?  thanks.</p>