Get email message-ID in Mail on iOS [duplicate]

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 View Source.

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.

OK. I think I've got this figured out.

First, you need to have access to a gmail account. Great if the original email is already address to one. Otherwise you'll need to forward to a gmail address (or an account that's accessible via Gmail).

Next, open Gmail in a browser on your iOS device. I used Chrome, but that may not matter. Go to gmail.google.com and sign in to your account. It will likely take you to the mobile version. Scroll down the page and you should see Mobile | Older version | Desktop. You want Desktop.

Once there, open the email in question. Just under the address info for the email, you should see Reply, Reply all, Forward.... Show original. Click Show original and you should be golden.