Spike in Undeliverable Emails from a User
This rule detects a significant increase in undeliverable emails for a specific user within a short timeframe. This pattern can indicate that a user's account has been compromised and is being used to send out a large volume of phishing or spam emails, leading to many bounce-back messages. It could also suggest a targeted phishing campaign where the sender's email address is being spoofed, and the legitimate user is receiving the undeliverable notifications.
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