Sometimes an email becomes a “thread”, sent back and forth and commented on by several people, while all the previous exchanges become "quotes" at the end (or worse, the beginning) of each new email. The quotes are handy if you forget what it was all about, but usually no one reads them ever, they just multiply. So, do you ever decide to edit them for succinctness?
A little voice may tell you not to change what someone else wrote, but you can make nice “…” marks to be honest about your surgery. Of course, when you notice that one of the quotes of your OWN text says something really stupid, you’ll just fix that without any edit marks, right?
And, I confess, I often fix other people’s spelling and grammar when I quote them.