In the file accounts.ini, each account (separately) will have a line that says Reply= followed by the text of the reply header (in English, it would be "On %d, %s said:" or something like that.) If you need to include newline characters (in the text version) then use escapes like in javascript: \n for newline. However, I don't know what it will do with HTML - it may include it as literal text. That portion of the mail client wasn't designed with HTML in mind, so it's 50/50 as to what it'll do with it.