This field contains the identity of the person(s) who wished this message to be sent. The message-creation process should default this field to be a single, authenticated machine address, indicating the AGENT (person, system or process) entering the message. If this is not done, the "Sender" field MUST be present. If the "From" field IS defaulted this way, the "Sender" field is optional and is redundant with the "From" field. In all cases, addresses in the "From" field must be machine-usable (addr-specs) and may not contain named lists (groups).