Vendors are replying to POs and RFPs by pressing REPLY in emails

I've asked for this one on numerous occasions… Most people press REPLY when the want to reply to an email. I have to train nearly every single client to press the green button to go into the studio to respond to their emails. Most are pretty good at it. Vendors, on the other hand are busy and tend to do things the same way to process every Purchase Order and Request for Pricing that comes their way. This means, they hit REPLY in their email for my POs and RFPs in. And I never see them. I have to follow up with them and ask them to resend. Why does this simple step need to be so complicated? I sent 3 POs to 3 vendors yesterday morning and they all treated the PO the same way, meaning their responses went nowhere. Even though they see this, it goes unnoticed. There needs to be a message that pops up to tell them the message cannot be sent and giving them instructions on how to reply. Please and thank you.