`smtpd_banner`

This parameter specifies the text that follows the 220 status code in the SMTP greeting banner. Some people like to see the mail version advertised. By default, Postfix shows no version.

You MUST specify the `$myhostname` at the start of the text. When the SMTP client sees its own hostname at the start of an SMTP greeting banner it will report a mailer loop. That's better than having a machine meltdown.