Be careful : There is a new native plugin Joomla that make redirections (if it is enable) actiaweb404 don't work ;) Thanks to Mario33 to report that.
To be sure the actiaweb404 work put it in the first place in the list of plugins
Actiaweb 404 is a free plugin that alert you by mail if there is an error http of your choice ( 4 http error is possible to check),
You can check 404, 403, 500 or others code you want.
The limit is 4 http codes but if you contact me i can add a better limit !
You have to enable the code you want
add your code : 404 for example
add your mail (if you want)
add a redirection (if you want)
in bonus : you can choice the subject of the mail and others parameters ... for each http error !
NEW : you have in the mail the ip of the bad visitor ;)