They call it TEN as in the Text Email Newsletter Standard (TEN Standard). It’s rules for formatting PLAIN TEXT emails (more than 10 haha). Here’s some I really like:
+05: Minimise any introductory or background text at the top of each issue, so that the reader arrives at the ‘contents’ section as soon as possible.
+09: Number all articles, including news and features, consecutively throughout the issue. These numbers should appear in the contents and then immediately after the + or # symbol at the beginning of each story headline, followed by a colon after the number and then straight into the headline.
+16: Try to list web links, email addresses or other internet addresses such as ftp addresses on a separate line, with just the address itself on the line, as this makes them easier to copy onto a clipboard or a browser, helps prevent ‘wrap-around’ or long addresses, and allows screen reader users to skip immediately to the next line if they do not wish to listen to the web address.
New bucket list thing is to list an FTP address in a public text newsletter.
I don’t know about this one. It would be *very hard* for me to stop doing it.
+03: Do not use bold, italics or underlining to convey tone or
information, as they are graphical devices, and are sometimes stripped out by email software. Try to convey emphasis instead in your choice of words; or by using phrases such as ‘Please note:’. The use of special characters such as an asterisk * on either side of a word to convey emphasis or for other tonal purposes is not encouraged, as this can be confusing to new readers.
I don’t think they mention line length, but I see the standard itself has hard line breaks at a certain line length. Personally I find that annoying, as if you’re reading *narrower* than that, you get this thing going on which is the worst.

As much as I want to love plain text email only all the time, for something like a newsletter, I prefer a very basic template so you can achieve stuff like basic good typography and links.
