This detection rule focuses on identifying Zimbra mailer emails that originate from non-supported versions of Windows. Specifically, it detects email headers to find instances where the Zimbra mailer starts with the identifier 'Zimbra' and uses regular expressions to search for specific version patterns (5.1 and 6.1) that are associated with unsupported operating system versions. This type of detection is significant as these emails have been linked to HTML credential phishing campaigns, making it a relevant security measure to prevent potential phishing attacks and protect users from malicious emails.