Update reports-email-security.md

add Mailbombing

Author: EHLOBen

defender-office-365/reports-email-security.md

@@ -562,6 +562,7 @@ In the **View data by Email \> Spam** and **Chart breakdown by Detection Technol
 - **IP reputation**: The message was from a source that was previously identified as sending spam in other Microsoft 365 organizations.
 - **Mixed analysis detection**: Multiple filters contributed to the verdict for the message.
 - **URL malicious reputation**: The message contains a URL that was previously identified as malicious in other Microsoft 365 organizations.
+- **Mail bombing**: Messages detected as part of a mailbombing attack (attackers flood targeted email addresses with an overwhelming volume of messages).
 
 In the details table below the chart, the following information is available: