If Lynis list the following warning:
Warning: found mail_name in SMTP banner, and/or mail_name contains ‘Postfix’.
You will fix it by editing the file
/etc/postfix/main.cf and removing everything after the ESMTP on the following line:
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
And of course, restart postfix:
sudo /etc/init.d/postfix restart