Graph API and non-Microsoft mail accounts

Question

Does Graph API work with non-Microsoft mail accounts such as Gmail and Yahoo etc? These accounts are already configured in Outlook using IMAP (or POP3). The add-in needs to iterate through the folders recursively and show user a report about matching mail items after performing some filtering on mail subject, body, attachments etc.

I was willing to take it up in VSTO, where this can be done easily, but the client is insisting on using Office.js so that they could port the add-in to Macs too. After spending a few hours, my impression is that:

1. There is an Outlook REST API that can probably work with all kinds of mail accounts, but this API is going to be retired soon and MS is asking people to switch to Graph API.
2. Graph API requires O365 accounts and cannot work with non-MS mail accounts.

Is my impression correct? If yes, is there an alternate way of achieving this in Office.js?