We're in the process of migrating all users to Outlook for iOS/Android. If corporate owned, they will enroll via the Company Portal app and have Outlook, Work, Excel, PowerPoint, Teams, OneDrive & Authenticator auto installed/pushed. If they are BYOD, I need them to only use Outlook for iOS/Android with an app password forced via Intune App Protection Policy.
I've attempted disabling ActiveSync, + OWA for Devices within O365. No luck.
Use myMail to manage messages in your Gmail, Hotmail, Live, Outlook, Yahoo, MSN, iCloud and AOL email accounts at once. Our app also supports any IMAP and POP3-enabled mailbox. myMail brings all your email into one simple, bright and friendly interface whether its your iPhone or iPad. myMail was
I had a similar situation, we blocked IMAP and POP using Set-CASMailbox, and now the new Set-Mailbox -AuthenticationPolicy. This doesn't strictly bind them to Outlook, but it prevents them from going out there and using non OIDC/OAuth based apps (like MyMail).
Like the other reply, you can use Conditional Access to to achieve a similar result and block the way apps like MyMail connect, but it won't lock your users choice down to Outlook only.
You can also use Cloud App security (license needed) to control business sanctioned apps.
@Brett Lindsey You should block legacy authentication anyway with conditional access. With that you get rid of most 3rd party apps. As far as I know, only the native iOS email application supports modern authentication.
Two policies with block as action control, one for other clients and one for active sync under client apps.
In combination with approved client app cond acc. and eventually App Protection policy you should be able to force the users to use Outlook