Thanks for that thought. Since I forwarded the email by hand from my email client, I didn’t send that header, but your comment did inspire me to look to see if there was in fact something simiilar in the error logs.
The error turned up in /admin/email/rejected:
undefined method 'lines' for nil:NilClass
Not sure if that’s helpful for someone more clueful than I.
Update: Another data point. I got the same rejection email and same logged error as above, when trying to forward in another human’s email. This makes me think it might be something more related to how the Microsoft Outlook/Exchange client is processing forwarded messages. Investigation on my end will continue.