Email / UUPlus
UUPlus is described in more detail at the UUPlus site.
UUPlus has an application which runs on macOS, and so using UUPlus from a Mac is pretty simple.
In order to access UUPlus from iOS, you can setup a network and have the iOS device communicate directly with UUPlus running on a Mac, Windows or Linux. There are a few steps involved in setting this up, but once that is done, using UUPlus is very simple.