The fix suggested by Apple Support in 2019 when this question was first asked is merely a diagnostic, not a solution. How do you wrap incoming text that does not fit in the window?
iMac 21.5″, macOS 10.13
If it's incoming text, then almost for sure the Mail is being composed on the other end with Non-Breaking Spaces...
Text-processing software typically assumes that an automatic line break may be inserted anywhere a space character occurs; a non-breaking space prevents this from happening (provided the software recognizes the character). For example, if the text "100 km" will not quite fit at the end of a line, the software may insert a line break between "100" and "km". An editor who finds this behavior undesirable may choose to use a non-breaking space between "100" and "km". This guarantees that the text "100 km" will not be broken: if it does not fit at the end of a line, it is moved in its entirety to the next line.
A fine suggestion, but it is easier said than done. I have no idea what email client or operating system the sender uses, the sender is almost certainly unaware of breaking vs. non-breaking spaces, what I find online about non-breaking spaces deals with line-at-a-time not global control from the sender's point of view, and I am looking for a solution for the recipient not the sender.
Of course, a simple work-around for the recipient is to Copy All and paste in Text Edit, which wraps according to window width. Apparently, Apple Mail just doesn't have this feature.
Consider asking, politely, of the Sender to compose without the Non-Breaking Spaces.
Thank you for your response, but I don't have any control over how the sender composes the message. In earlier OSs (c. 2014), you could simply open the window wide enough to get to the end of the line, double vertical lines with a left arrow would appear, and the right border could be dragged to the left until a suitable window width was found. When composing a message currently, you can drag the lower right corner of the pane to get the same effect. Neither of these works for displaying the incoming message with long lengths to fit within the pane.
Was just about to suggest that workaeound.
My bet is they're using Windows® & Word®.
MacOS High Sierra- mail text not wrapping automatically
