Cannot retrieve message ID

I have a suitescript that sets case statuses based on incoming case reply emails.  In order to do this, it is attached to the message record as a post submit script.     It worked fine for a year and then suddenly stopped working some time in the last month (hard to tell when, since it's not used continuously.)   Using some log statements, I found that the line:

var ID = nlapiGetRecordId();

now always returns "" or null.   This of course means I can't use the script anymore.

Anyone have any ideas on what has changed or what is going wrong?   I know that operations on message records have been sketchy, but my business depends on this functionality.