Send mail with custom headers

Hi all, I have done a function to read the email from my application using imap function in PHP and it is working fine. but after retireve I want to send a reply to the same mail thread. I tried to achieve this by setting the in_reply_to and reference values in the header while sending the email using the withSwiftMessage function in the mailable class but I am not recivieng the mail after add the custom header. below showing my code. mailable class