Hackers somehow using mail_php.php

We all fight Spammers using our forms to send spam as Contact emails.
We use reCaptcha, honeypots and anything else we can think up to thwart the automated spamming of our Customers.

Now I have a spammer sending daily emails which look to come from the mail_php.php file.
How they are doing this is beyond my comprehension.

1st I saw some of my other customer get spams and one noted "testing XRumer" in one of the fields. Cool, I'll research this to see what they can do.
Turns out a lot. XRumer can bypass reCatchas and HoneyPots.

So I decided to change the comment field id from "message" to "34rrSGD" and make the honeypot field "comment".
Next our code says IF comment field has values ignore sending an email and setting the post as a DB record.
Lastly we change the title for column 34rrSGD to Comments in the final email.

There is no longer any field named "message".

BUT we are still getting emails sent to the Customer in the original email format and using the "message" field.

We DO see legitimate Contact Submissions posting to the DB and also swapping out 34rrSGD to Comments.
The "Blank" template being assigned to the email response has been changed. We now hide the hidden consent field ($remove[] = "hdn-consent";)
There is NO message input
and we swap out 34rrSGD with Comments
if ($pkey == "34rrSGD") $pkey = "Comments";

Yet there is an email generated multiple times per day, coming from our server, using the old non-existent template.

I can only think that the Bad Guys somehow also send themselves a copy of the originally formatted email response and are now resending that but with different content each day.

Problem is the mail server logs show it's coming from our server AND the email headers are saying it's coming from mail_php.php
"X-PHP-Originating-Script: 0:mail_php.php"

I've attached a screen shot showing the spoofed email content and the latest legitimate one.

How could this be happening?