Replying to emails - spacing problem

[Helenh]

When I reply to an email, if I want to add to the text already there, when I
type it adds in a space before and after the letter/word I'm typing.

For example a sentence would be "the dogs sat on the mat" and I want to
change the word " mat" to "mats". It would look like this: "the dogs sat on
the mat s ".

Is there a setting somewhere I can change to stop this happening?

Many thanks
Helen