Where is the incoming mail banner (below) set in MailScanner? -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.
The banner is set in postfix, not mailscanner. It can be found in /etc/postfix/main.cf. The message you're getting is the called the 'Inline HTML Signature'. This is only true if the 'Sign Clean Messages = yes' option is set. This can be found in the /etc/MailScanner/MailScanner.conf file.
I don't mean the smtp banner but rather the one displayed in the received email saying it has been scanned and believed to be clean. <edit> Found it in /etc/MailScanner/reports/en/inline.sig.*