After blindly raising errors for 'SMTPUTF8' without 'BODY=.*' I found out that 'SMTPUTF8' alone is actually perfectly fine (for using international names in addresses but sending ASCII data). Which means I did a mistake in the smtpd patch :(
The attached 3rd version of the smtplib patch implements your suggestions, including re-factoring of the mail option parsing. One test is commented out until smtpd is fixed. I'll do that as -part of- issue 21795 tomorrow.