As long as webmaster_email is a field type that is case insensitive (per the MySQL manual), you can use:
$row_query = mysql_query("SELECT DISTINCT( webmast_email) FROM $toplist_mysql_table");
This will return each e-mail address only once, regardless of case (again, assuming that the field is a case insensitive field type, such as char or text).
Hope this helps!
-Rich