To me the Undisclosed Recipients - would make me think Spam with multiple recipients in the BCC field. Combine that with no Subject line and my Mk1 brain would treat the email as suspicious. Heuristics would probably do the same.

Try getting him to enter a Subject line and direct the email to you (in the To field) not Undisclosed Recipients (leaving the To field blank), rather than put your email address in the BCC field. Unless of course he is sending it to multiple recipients, in which case he should put a dump email address (hotmail, etc.) in the To field (this account can be set to delete all content).