We have a particular sender whom occasionally sent an email to our domain. Whenever any of my users reply, it creates double space paragraph. We tried Format Text ribbon > Change Styles > Paragraph Spacing command. Set as Default after changing the spacing. However, this is annoying for all users do to every time. We are looking for a permanent solution not work around.
So either change the format (as you say you know how) or request the sender to fix their style. If they are decent they might do this as double spacing for a paragraph is not Word default. Outlook uses Word as its editor.