My understanding is roughly, for example:

• Microsoft Word desktop application: not SAAS.
• Microsoft Word online: SAAS (just like any other service accessible by browser but not a “localhost”)
• Onedrive: SAAS, storage with local explorer integration.
• Exchange server on prem: not SAAS, increasingly diffucult to do.
• Exchange server by MS: SAAS
• Microsoft Outlook Classic for desktop: not SAAS.
• Microsoft Teams for desktop: SAAS although local install but its just another frontend instead of browser.
• Office365: SAAS but really a container for every tool in the MS online toolbox together.

Some caveats: Word handles spellchecker in their cloud and clippy 2024 (Copilot) integration blurs the line.