It's not a solution for iOS, but Fair Email is an open source Android mail client that fits this description. It can work as a text only client for sending and

It's not a solution for iOS, but Fair Email is an open source Android mail client that fits this description. It can work as a text only client for sending and receiving, and defaults to pretty much everything being safe. Links are hyperlinks, but bring a pop-up dialogue which shows you the destination, and will even unmangle common trackers from links, or MS Outlook advanced protection mangled URLs. It pretty much defaults to being this "reduced attack surface", and I wish more apps were like this. It's probably one of the more complex settings UIs you'll ever find in a mobile app, but that's because almost everything is able to be enabled or disabled... Want it to check and validate DKIM locally? That's a toggle switch. Want it to auto remove tracking pixels from html emails? That's a toggle. Want to view email from trusted contacts in html view? Pretty sure that's a toggle too. No connection, just a happy user of a nice open source email client.