I don't see why even K-9 does this but Proton won't. Is it a privacy-related thing?

Full explanation follows, for all it matters.

New to Proton, I use it on several devices although my main is a desktop computer, from which I keep track of every single thing going on at my company. The desktop is obviously locked up and safe, but my mobile devices, being mobile, are with me most of the time. Should either get stolen, I wouldn't want the info on it to be way too important - which is why I don't use it as a credit card either. Stuff like that, that many Proton users probably think of sometimes since they're privacy-conscious.

I've used Thunderbird, K-9 and Mail for a while, and I can always BCC myself when sending emails from my tablet or phone, so that I can always double check that I did send an email, even when away from the device.

I guess SMTP is indeed a solution many can make do with; however, sometimes I don't need an email to be on my phone or tablet, but I do need it to appear on my desktop client.
Why?
Convenience and confidentiality.
When I receive confidential information from clients, I immediately delete those email from my portable devices just in case I drop it or it gets stolen. But I still need to read those email on my desktop.

So, all this mess could be at least partially solved with an "Always BCC myself" function. (The no-POP3 is still an issue but I'll address it somewhere else)