A spammer is using a contact form on my website to send relentless spam. I have check my contact page and the spam is not coming from there.

Looking at the spam email headers show that it is coming from /public_html/index.php which is just wordpress - so I'm thinking there must be an old contact form somewhere on my huge 20 year old site that I can't find, but that spammers are using.

The email subject line is the only clue I can find and all it says is "Subject: New Message From" and then my site name.

I tried downloading the WP database and searching for that subject line, but it was not found.

How can I find it this form on my site?