It's often down to whether your e-mail client (or web service) sends them as "plain text" or "HTML" (or both).
When some message is sent as HTML, it will often (as with a web page without specific width controls) open to the width of the window in which it is being viewed. With widescreen displays on home PCs and laptops becoming more common, my guess is that your e-mail contacts are opening the mail on widescreen displays.
Problem is that humans find it quite difficult to keep track of a horizontal line once you exceed about 100 characters (well, the difficulty is going onto the next line, so one loses place and need to re-read or do more concentrating).
If you can send your message as plain text, and hit the enter key when you exceed about 70 characters then the message will be tied to that width even if they have a really wide screen to view it on.
If you have no option to set 'plain text' then pressing the return/ enter key should be OK (but because under HTML the assumption is that you type until the end of the paragraph and then press enter, so each line would be considered a new paragraph, and therefore the mail could look as if it is 'double spaced').
Cannot think of a way around it. I sometimes find my lines of text are displayed as a line and a half (and I do try to keep to 70 characters whenever possible) which just makes the whole e-mail look untidy (and perhaps makes me look like a fool too).
Some mail applications block HTML - because it can be used to download malware or include images from some website, where the fact your mail s/w has called that image means a spammer has confirmation (a) the e-mail address is valid, and (b) that the human was willing to look at the mail... leaves the human/ e-mail address open to getting much more spam as a result of that simple confirmation. (These are called web bugs or web beacons in case you are curious.)