Currently it is possible to use Launch in order to Launch("mailto:") to launch Outlook when creating a new email message, however there is a limit of 2048 characters in the entire "mailto:..." URL.Adding extra characters, e.g. too many mail addresses prevents Outlook from opening and no error messages are seen.
Using Office365.SendEmail means editing in a PowerApps screen and then sending, which means that the formatting abilities, proofing tools etc. are unavailable to the user without overcomplicating the design.
If a new function Office365.CreateEmail was available then the user could create any variables or collections containing the mail addresses from a list and then the function would open Outlook with all addresses added and could then add or delete addresse, add the body text, perform formatting etc. from within Outlook.
I have tested Office365.SendEmail and verified that I can send to 266 addresses from PowerApps using the function (no idea of the current limit here) but as already stated the mail design needs to be within PowerApps. With Launch(mailto and a short subject line I was able to launch Outlook with 80 email addresses maximum and then send from Outlook. The new function would avoid this limit by not launching Outlook via a URL.
It should be added to the Office365 and Outlook Connectors and would also then be available from within MS Flow.