What is the purpose of sending an email to an invalid address in a penetration test?

• A. To verify the recipient's mailbox exists in real time.
• B. To test if the mail server scans for malware in attachments.
• C. To measure email delivery speed.
• D. To gather information about the email server's handling of undeliverable messages.

Answer: (D)

Observing how a mail server handles undeliverable messages when you send to an invalid address reveals valuable information about the email infrastructure. When the address doesn’t exist, the server issues a bounce (a non-delivery report). Analyzing the bounce content and headers can show what mail software is in use, how verbose the error reporting is, whether a catch-all is configured, and how much internal detail is disclosed in the responses. This helps map the mail system and identify potential misconfigurations or information leakage that could be exploited. It isn’t about confirming mailbox existence in real time, malware scanning, or measuring delivery speed, which aren’t related to how the server handles undeliverable messages.