Marcel, Tony, Tobias, thank you for the info. I had this notion that using text/html would handle this sort of thing automatically and clearly... not.

I seems like 

	[message := MailMessage new
		from: senderName;
		to:  recipient;
		subject:  subjectString.
	message body: (MIMEDocument contentType: 'text/html' content: anHTMLString base64Encoded).
	message
 		setField: 'content-transfer-encoding' toString: 'base64';
		setField: 'mime-version' toString: '1.0'.

solves the problem. Yay!

tim
--
tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
"Bother" said Pooh, as his trigger finger tired.