Perhaps you could tell us what application you are using and whether it is
text based or html based email. This often happens when one person is using
text based email and another is not. The hard returns are embedded in the
text because the text based programme was set to a fixed width, usually 80
characters. I usually get around this by pasting without formatting which is
a Thunderbird option when you right-click before pasting. then you have to
add your own formatting, but at least you do not have to strip out old
formatting.