I forgot something, using sed will show the result as stdout, so it's necessary to direct it to a file:
sed 's/expression/other expression/g' file > other file
So It should be easier with a shell ( could be used with email addresses too, and subdirectories
) :
# for file in `find . -iname *.html`; sed 's/whatever@whatever.com/whontever@whatever.com/g' $file > $file.new ; done