[mailop] VERP generating syntactically invalid return-path?

A contractor just started getting bounce messages when he tries to email $DAYJOB -- the message is accepted by our Exim frontend but then rejected by our Exchange 2013 server with the error message below (example.com and example.net are placeholders for two different .com domains; outside sender

Re: [mailop] VERP generating syntactically invalid return-path?

On 2/2/16, 7:59 PM, "mailop on behalf of John Levine" wrote: >>return-path address