Do shared mailboxes need a license?

Up to 50 GB, no. A shared mailbox in Exchange Online does not require a license as long as no user signs into it directly — staff access it as a delegated mailbox from their own licensed account. Above 50 GB the shared mailbox must be assigned a Microsoft 365 license (typically Exchange Online Plan 2).

If you need In-Place Archive on the shared mailbox, that also requires a license. Same if you want to enable Litigation Hold for compliance — those are licensed features, even on a shared mailbox.

Still unsure?

Talk to a licensing specialist for a written, signed quote tailored to your environment.