Its 2021 and its still not possible with the built-in app! Nor with Outlook.
Thankfully Airmail does, even the free version. Open an email, tap the
... at the bottom (not the top one as that one didnt work for me) and then
Note though that the free version will keep prompting you to purchase a subscription so its not a substitute. Also, and more importantly, the text size is quite small and the app doesn't allow you to zoom. You can copy the text if you must so there is that at least.
The second part to your question, sending email as attachment, Airmail does this as well.
When I tested sending as attachment from Microsoft's 365 (using Airmail) to GMail, I couldn't view the attachment in either built-in Mail app or through the web browser. I got a message that emaill could not be downloaded. When I added my GMail to Airmail, I was able to open the attachment.
Airmail to Outlook also worked in my tests.