Answer
The username-based overload never supported a mail-sender parameter, even going back through Apps10 — for specifying a sender, use one of the sender_/from_/to_list_-based overloads instead (which take a CLOB for the message body rather than a plain string); one variant accepts a single CLOB attachment, and a separate variant accepts an attachment_arr for multiple attachments — pick whichever matches whether you need to send one attachment or several, since both support the sender parameter the username-based overload lacks.