Problem associating an email to a customer record

Hi there and thanks in advance for any help or advice.

I have a script which automatically sends out overdue notices (with statement attachment) to customers - simple enough.  I have not been able, however, to record the email (and contents) in the customer's "messages" tab.

I have searched on the forum, and despite all the previous samples of nlapiSendEmail, no luck yet.

Here is the code that I am currently using - for troubleshooting and testing, I am using the Anonymous customer account (internalID of -6):

var records = new Object();
var StatementAttach = nlapiPrintRecord('STATEMENT', CustID, 'PDF', sdate);
records['entity'] = 'customer';