Unable to open EML files on iOS devices

On our iOS devices, we cannot open .EML files. We were told that iOS requires the Apple Mail app to open .EML files natively.

We currently use Microsoft Outlook for sending and receiving email and for calendars. Outlook works fine for mailbox functions, but it does not appear to open .EML files on iOS devices.

Is Apple Mail required on iOS strictly as the system email viewer for .EML files, even when Outlook is the primary email app?