Er, that's how it works. (You can look up all the RFCs if you like, but you'll soon get lost in a tangled web of incomprehensible gobbledegook, I know I would and I get paid to find my way round that stuff on a bad day.)
(1) You can send emails as plain text. (Which is usually best for lots of reasons, not least of which is that some people set their servers to silently drop all HTML email because 9.99999% of it is spam.) Lines will get reformatted by something or other somewhere along the line.
(2) You can send emails in HTML format. Lots of people don't like this ... but if your recipient doesn't mind, then any reformatting of the HTML code during the transmission process doesn't matter, as the display is being done by the browser at the recipient. However this way what the user sees is, particularly in terms of line lengths and so on, is down to the window size they've chosen and the browser they're running and various browser options they've chosen.
(3) If you want the recipient to see something exactly as you send it you've little choice but to send your email as an attachment in some suitable document format, PDF would be the obvious choice. But do put some text in the body of your email explaining what the attachment is, otherwise the recipient will quite likely delete it as a probable virus, and don't seen emails as PDFs unless it really makes sense to do so, eg it's arguably for a society's monthly newsletter, it's *not* ok for an "I can do next Tuesday".