"Department" is a drop down where the value is an email address and the text is the department name. When placholding $$value:Department$$ in the email or even putting {%department%} in the Subject box it indeed produces the email address we want.
Thanks for the debug suggestion, it has produced the following which I'm researching but theres not much out there in relation:
Error while evaluating expression: Department
CMS.MacroEngine.LexicalAnalysisException: Lexical analysis of the expression failed. Error near: ->>Department
at CMS.MacroEngine.MacroElement.ParseNext(String expression, Int32& index, Boolean supressError, Boolean isParamValue)
at CMS.MacroEngine.MacroElement.ParseExpression(String expression, Boolean supressError)
at CMS.MacroEngine.MacroExpression.ParseExpression(String expression, Boolean supressError)
at CMS.MacroEngine.MacroResolver.ResolveMacroExpression(ResolveExpressionSettings settings)
Just seems very odd that this field works when using it as the selected field for "Confirmation email source field" and also works fine as {%Department%} in the "Subject" field, but the one place I need it it's throwing the above error.
I'll keep investigating.