The issue is being caused by iPhone/iPad users using the native mail app on their device and the workaround is to use Outlook for iOS.
We've thought about putting a block on the native mail app from connecting but that seems really heavy handed and trying to get everyone to use the Outlook iOS app may never happen.
If anyone else is in the same position I'd like to hear from them and if they have any other suggestions. We've gone as far to raise a premier case with MS and also asked our Apple rep to get talking to one another to get this fixed as we see these phantom invites go out weekly :(
Yeah totally. We have asked people to use the Outlook app but getting people to make that their default app can be tricky. I did think about if we should see if we could block the native app from connecting but we don't like the thought of doing that right now as it might cause more aggro.