I figured it out - it had to do with the email configuration within the plugin.
Reply-to email within the form needs to be sent from an address on the hosted website. I had seen some talk of this somewhere else within the support forum, but hadn't understood it clearly at the time. Easy fix, in the end.
Would have saved me hours, though, if the '?' had just stated that it should be an address from the website; it would otherwise not make sense to me to use that address (an address I don't use at all).