This seems like a very limited case, but here’s hoping 
I subscribe to a couple of email list digests, and the emails come in as .eml attachments.
I’m looking for an iOS app that will open .eml attachments, this so I don’t have to go to the Mail app and open the emails with .eml attachments there.
On the few occasions where I needed to open .eml files, Drafts happily accepted them.
Perhaps through Letter Opener (Appstore).
Thanks. Tried that, but these are html, and not very legible.
How about just looking at them with QuickLook?
Somehow I did not see that you were looking for an iOS app. I’d be curious to see if there is an app out there that can do this. This just underscores how many things that are so simple on macOS are such a chore on iOS. Yes, iOS has the “delight” factor, but when will it catch up in being able to do simple tasks like this simply and easily?
Thanks for the earlier reply.
I found one app that was supposed to do this, couldn’t get it to work, developer didn’t respond, got a refund. Developer responded. Hopefully a learning experience for him or her.
As far as macOS, Spark has the same problem. If I want to see what’s in a digest email, I have to use Mail.
I just found out how to create links to Mail messages, if I had snooze capabilities in Mail (show me an email later, let me know if no response), I’d just go back to Mail fulltime.
It seems my (and others’) standards for what an email client should do are far outstripping what developers are making.