Yes and I opened a new thread at Mail binding errors sending mail UnsupportedDataTypeException: text/plaijn; charset=UTF=8.
Are you by chance running with JSScripting or using a different rules language?
It’s probably prudent to have most of the discussion over on the thread I started though.
Circling back, I’ve opened an issue for this: [Mail] sendMail and sendHtmlFail errors for UI rules · Issue #11838 · openhab/openhab-addons · GitHub It seems to impact all UI based rules regardless of language.