I administrate an email system for a commercial ISP. I'm currently having an issue with one of our customers relaying outbound SMTP traffic from their Exchange system through ours, and they're generating NDRs and other failure messages in response to spam. I need them to stop doing this, however their own admin is not very savvy. I'd rather provide them some guidance than leave them blocked from using our relays.
Having never worked with Exchange (I'm a *nix/BSD guy), is there some way they can accomplish this without other software or hardware in front of it? I've looked at the header content and they are scanning for spam, which is adding an X-* header which identifies the message as such. Can Exchange outbound filter this?
Does anyone have any other ideas on stopping these NDRs before they ever get to our servers?