Delegated vs App-Only Auth in Microsoft 365 Migrations (Hard Lessons)
cloud•Jan 8, 2026

Delegated vs App-Only Auth in Microsoft 365 Migrations (Hard Lessons)

When migrating Microsoft 365 tenants, it's essential to move beyond delegated authentication and consider app-only authentication for its benefits in scalability, security, and reliability.

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Key summary

When migrating Microsoft 365 tenants, delegated authentication may seem appealing due to its ease of setup and debugging. However, it has limitations, particularly when it comes to user-based throttling, parallelism, and background jobs. App-only authentication, on the other hand, may seem boring but offers better security, more honest permissions, and improved retry logic.

TL;DR

  • Delegated auth has limitations in user-based throttling, parallelism, and background jobs.
  • App-only auth offers better security and more honest permissions.
  • Retry logic behaves differently and better with app-only auth.
  • Delegated auth is fine for small-scale migrations or demos.
  • App-only auth is a prerequisite for large-scale tenant migrations.
  • Most migration failures come from wrong assumptions, not bad code.
  • Tenant migrations require app-only auth for reliability and scalability.
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