DKIM Signature with email function

Hello,
I'm using CI4 for a production project, and more and more emails sent through CI email function are being rejected because they are not signed with a DKIM signature.
I have these settings in my .env file

PHP Code:

email.fromEmail = 'xxx'
email.fromName = 'xxx'
email.mailType = 'html'
email.SMTPHost = 'xxx'
email.SMTPUser = 'xxx'
email.SMTPPass = 'xxx'
email.SMTPPort = '465'
email.SMTPCrypto = 'ssl' 


What should I add to use my DKIM private key here ?
Thank you