Hi Zach and thanks for the reply.  I have already adjusted Outlook to send as Plain Text.  I did that a couple of weeks ago, I just had to wait until yesterday when the bulk invoices went out.  As I said in the OP, we couldn't test this until now because sending a single invoice to this customer is received properly.  That seems to indicate that the Outlook settings are fine.

I will check into the spam filter at the customer location (thankfully we're an IT services provider, so we can check our client's settings pretty easily).  If I find anything there, I'll update.