The elements of typographically correct email

This subject has been sitting in the combustion chamber for a while and I need to get it out on the nets.
Email applications need to allow for specifying line width for text. When they don’t, the amount of words/characters per line is dependent on the size of your message window. It shouldn’t be the responsibility of the user to have to adjust their email message window to the width for ideal readability. Computers, and by extension, their applications should reduce complexity, not add to it.
When email applications let line width run wild, it makes it much harder to read messages as your eye has a much farther distant to snap back to get to the beginning of the next line, increasing the likelihood of a reader losing their place.
From The Elements of Typographic Style as Applied to the Web (referencing the original EoTS):

Anything from 45 to 75 characters is widely regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal. For multiple column work, a better average is 40 to 50 characters.

While this statement was referring to printed text, the same principles apply to line width for the web. Everything is relative and when designing for the web and it’s up to the designer to determine the ideal type size combined with the ideal line width (this site uses a main content width of 545 pixels against a font size of 12 pixels).
Click on the two screen grabs below to compare readability (viewed in Entourage):
plain text with imposed line breaks
rich text with no line breaks
The only application on OS X I’m aware of that allows for changes in line width is Entourage. In Entourage, when you switch from rich text to plain text, it automatically resets the line width to 76 characters through the use of line breaks. In Apple’s native, it does nothing to line width when toggling between rich & plain text.
I’m not sure what the solution to this is, since with Entourage’s method of imposing line breaks into the previously ‘plain’ text causes situations such as this on smaller screens, like my iPhone:
Those previously wonderful, strategic, 76-character long line breaks now look like shit on my 320-pixel wide iPhone screen.
The concept of liquidity of content is an important one when designing for the web and other digital content that lives in different environments and is interpreted by different applications. But too much liquidity means not enough structure.
…which means poor readability.
..which means poor communication.
What I suggest is that email applications (both local and web, like Gmail) impose a width on messages by default. If a user doesn’t like the default formatting, they should be able to adjust or remove it.