Does no responses indicate that this is not possible? I would be surprised, but that seems to be the indication.
There is no simple way to do it that I can think of.
You could import into Table A and use a relationship based on email to match to records in your main table. Then a looping script or Replace Field Contents step might be used to update the timestamp field when comparing values in the two related records indicates that the value will change when the data is imported.
Thanks for the feedback Phil. I had seen you post similar responses to other, older posts and was hoping that maybe something had changed in newer versions of FileMaker so that there might be a method to accomplish this without creating additional tables and relationships and complexity. I guess not. Thanks for the response, I appreciate at least knowing that.