About once every week or so, someone starts trying to find a mail resource on my site by typing in likely paths such as:
/cgi-bin/formmail.cgi
/cgi-bin/formmail.pl
/cgi-bin/FormMail.pl
I get 404 notifications for several similar URLs until they apparently find the right path. Next, I start getting mail returned to me that I did not send. (The "from" address is not mine but my server is set up to send any mail not at a real address to a certain account.) The headers are always screwed up, none of the "to" or "bcc" addresses are valid and the message body is unintelligible. Obviously, they are trying to hack my mail server and use it for some nefarious purpose.
I contacted my ISP and they said "Don't worry about it. It's impossible for them to send mail this way..." Well, if this were true, why is this attempted so often? Is this a well known hack and how can I guard against this?
Any insights?