You've got it, FileMaker has no way to tell if the email program sent it or not. You might have the script pop up a dialog after the send mail step that asks you "was email sent?" or some such confirmation and when you click the "yes" button in the custom dialog, your script can then update the field.
Hi Phil and David
Thank you both for your feedback; much appreciated and helpful.
I may not have understood the request perfectly, but the following script worked fine for the use I wanted. (It works at least with Outlook):
Set Error Capture [On]
Send Mail ...
If [ Get (LastError) ≠ 0]
Set Field [Field name; get (currentdate)]