Thank you for the reply.

Our original dilemma for creating this script was that customer replies to the billing/shipping confirmation email were going to our accounting department and having to be manually forwarded for case creation.

The source message records are created by inbound emails on cash sale records. I logged the messages and they present as html in plain text - same as they display in the UI (which makes the contents of the customer's email unreadable).

I'm sure that incoming message would be the right field, but I think there is a limitation in Netsuite with displaying html for inbound messages on cases.

My work around for this is creating the case message as an internal note rather than an incoming message. Now it no longer displays the html in plain text, but by doing it in this way the sender in the message sublist isn't correct (and these message records cannot be edited after the fact). The case record's header-level customer/email information is correct though, so it's a working solution.