Unexpected Microsoft Defender for Office 365 License Requirement for Shared Mailboxes

A question about shared mailboxes brought up the topic of licensing requirements when a tenant has Microsoft Defender for Office 365 (MDO). The news is not good. Once MDO is active, every shared mailbox needs an MDO license, and every user mailbox must also be licensed for MDO (those with E5 licenses are covered). At $5 per month, those MDO licenses can ramp up to a considerable cost. Ouch!

https://office365itpros.com/2025/08/11/microsoft-defender-for-office-365/