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.