also if you are using your own PHP script to send what is the format of the send message.
for example, the headers are important, who its from and ties in with the sending mail. also setting a valid and good constructed Message-Id is important now (I only included this in lasy year or two)
also I check the incoming URL for hackers and spammers. as soon as I see something along these lines I ban their IP address, so they only get one chance (per IP address)
yes, its an automatic system. there are quite a few checks on my list.
yes I can tell how frequent they try.
so far so good keeping them at bay, but its constant so need to regulary check the logs for new tricks