There is not enough information or clarity posted to identify an issue. It sound like you're saying each individual is using a script to generate a message (to the group?) and then you say the script reads the email and processes the contents. This sounds very convoluted and confusing!
That said, the single most common reason for bouncing is spam prevention on the receiving end where there is a perceived issue with the content or the source of the message. That's where you start but, sometimes, little or nothing can be done about it.
If you're including or attaching a script to a message, I would not (and I do not) allow it as an email administrator.
Also, you cannot rule out a script simply because you believe it "has stood the test of time." I would be extremely skeptical of any script written 10 years ago.
I strongly urge you to consider doing – whatever it is you're trying to do – in a way that meets currently-accepted email security and validation practices.