Email aliases: How to differentiate between an internal alias and an external one?

Question

So an Outlook account allows for aliases. It allows for both internal aliases that don't exist outside of the account and simply route straight to the same inbox and for external email accounts to be used as login usernames and to send as.

The thing is that I cannot find any way to differentiate between the two, at least not definitively. This has to be doable on both business and MSA accounts. I can get all of the emails either by $select=proxyAddresses on /me call for business accounts or (since it's the ONLY way to do it) by using the beta /me/profile call to get them for MSA accounts. Neither one gives me enough information to reliably tell which is an internal alias and which is an external. Sure, there are cases where it can be determined, like extra outlook.com addresses on an MSA account, but not always, and this may not hold true for various older grandfathered configurations, either. Even an MSA account can have a custom domain through O365, so going by domain alone can't provide that information. Even MX records may not be reliable since they can be edited.

Does anyone have an answer to this issue? What I'd give for Microsoft to simply add a flag to each email address in the API results identifying the type...