[userbinator]

Text-only emails can and will still contain links, which users will still click on. Misleading domain names will work just as well in the email body as they do in the address bar.

...that brings up the other elephant in the room: Unicode. More specifically, https://en.wikipedia.org/wiki/IDN_homograph_attack

The 0/O/i/1/l/I distinction is a classic one that can be somewhat mitigated by good font choice (plaintext emails are often presented in monospace, so that helps a little), but some of the other sets of Unicode characters are designed to look identical to ones in the ASCII range, often using the exact same glyphs.

For those who use non-English characters exclusively, it is not possible to use ASCII only emails, but non-Unicode encodings are still helpful in this situation --- e.g. the Cyrillic characters most known for IDN homograph attacks aren't even encodable in 8859-1 or Shift-JIS.