This rule is designed to detect phishing attempts that leverage email addresses originating from the onmicrosoft.com domain while being sent through Sendgrid, a cloud-based email service. The rule flags emails that exhibit this unique combination of properties, which is often exploited in credential phishing attacks. Specifically, the detection logic checks for inbound messages where the return path domain is 'sendgrid.net' and the sender's domain is 'onmicrosoft.com'. Furthermore, the rule excludes common sender email local parts associated with legitimate administrative emails, such as 'postmaster', 'mailer-daemon', and 'administrator', to minimize false positives.