call plugins to process subscribe page and return error if a plugin returns non-empty value
call plugins to process subscribe page and return error if a plugin returns non-empty value

mariez/phplist3,byronjumbo/phplist3,phpList/phplist3,samtuke/phplist3,mariez/phplist3,danwaterloo/phplist3,byronjumbo/phplist3,bramley/phplist3,bramley/phplist3,phpList/phplist3,samtuke/phplist3,bramley/phplist3,tarekdj/phplist3,mariez/phplist3,tarekdj/phplist3,samtuke/phplist3,phpList/phplist3,danwaterloo/phplist3,tarekdj/phplist3,phpList/phplist3,danwaterloo/phplist3,byronjumbo/phplist3,mariez/phplist3,samtuke/phplist3,danwaterloo/phplist3,byronjumbo/phplist3,tarekdj/phplist3,byronjumbo/phplist3,mariez/phplist3,phpList/phplist3,samtuke/phplist3,danwaterloo/phplist3,bramley/phplist3,tarekdj/phplist3,bramley/phplist3