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.
