This is a trivial question but I want to understand how spam works. On my site I get a contact us form submitted every night between 2:30am to 3:00am. There is not message in the form and I know it is a spam. But my question is what type of spam it is? Why not post something when you submit, whey not post it multiple times, why just one time, why only at this time of the day? What type of spam it is? Are there different of of it?
An automated bot may be using your script to send spam out to other users. You may only see one email but using header injections they're spamming countless other people. Besides putting in the usual anti-bot mechanisms make sure you also filter out unwanted content from the user submitted content to prevent the injections which can still happen from manual submissions.