Can I mix Microsoft 365 E3 and E5 in the same tenant?

Yes. Microsoft 365 plans are per-user licences, not tenant-level SKUs, so E3, E5, F1, F3, and Business Premium can all coexist in the same tenant. Tenant-scoped services (Purview policies, Sentinel connectors, Defender features) run at the level supported by the licensed users touching them.

How the mix actually works

• Give E5 to users who need the specific E5 workloads (Defender for Endpoint P2, Purview eDiscovery Premium, Advanced Audit, Power BI Pro, PIM).
• Give E3 to users who need the productivity stack without those extras.
• Give F1 or F3 to frontline staff who qualify.
• Business Premium and Enterprise plans can coexist up to the 300-seat cap on Business plans; above 300 seats the SKUs must be moved to Enterprise.

Where the mix bites

• Some E5 features require the *content owner* to be licensed, not the person acting — e.g. auto-labelling processes a mailbox only if the mailbox owner is E5.
• Auditing detail level depends on the user being audited being licensed for it.
• Purview retention and DLP policies can target unlicensed users, but the enforcement point may fail silently — check licence coverage before assuming a policy is active for everyone.