Outlook mobile still redirects to Google after migrating mail to Microsoft 365

This sounds more like an autodiscover or client auto-detection problem than an MX record problem. MX only controls mail delivery; Outlook mobile can use autodiscover and cached account information to determine where authentication should happen. Check every autodiscover-related record and any remaining parent-company configuration, then remove and re-add the account after clearing the app’s cached account data.

Since autodiscover.ourdomain.com resolves to autodiscover.outlook.com, the public CNAME itself looks correct. I’d test the affected domain with Microsoft’s remote connectivity tools and verify that there are no leftover SRV records, HTTP redirects, federation settings, or alternate autodiscover endpoints. Also make sure the users are adding the Microsoft 365 account with the correct work address rather than an old Google account profile.

Having a separate domain is important here, so it is less likely to be a namespace collision with the former company. If webmail works but mobile setup does not, focus on Outlook mobile’s stored discovery data: remove the old account from the app, clear its app data or reinstall it, and then add the mailbox again. Existing mobile profiles can retain the previous Google authentication route even after DNS and Microsoft 365 settings are fixed.