The detection rule identifies suspicious emails that are received from Gmail servers, which exhibit atypical sender characteristics associated with malicious behavior such as callback phishing and spam. By analyzing inbound emails, the rule targets those where the mailer is identified as common mail-sending libraries such as 'Microsoft CDO for Windows 2000', 'PHPMailer', or 'nodemailer'. Additionally, if the email headers show a hop originating from 'smtp.gmail.com' and do not have known false positives, it flags potential malicious intent. This approach allows for effective filtering of emails associated with phishing tactics and general unsolicited spam, leveraging header analysis to detect unusual sending patterns.