Also known (confusingly) as an "email sniffer." Message format which includes both an HTML and a text-only version in the same message. Most (but not all) email clients receiving messages in this format will automatically display the version the user's system is set to show. Systems that can't show HTML should show the text version instead. This doesn't always work, in particular for many Lotus Notes users. Also, no data, except HTML open rates and possibly link click tracking, is transmitted back to the sender regarding which version a recipient wound up viewing.