When sending out a single email salesforce encodes the HTML body
setHtmlBody(String)
Optional. The HTML version of the email, specified by the sender. The value is encoded according to the specification associated with the organization. You must specify a value for setTemplateId, setHtmlBody, or setPlainTextBody. Or, you can define both setHtmlBody and setPlainTextBody
Where is this setting in a dev org to control the type of encoding?