Do you mean RFC822 (the internal headers you see?) or the SMTP RFC821 head called MAIL FROM? As far as I know a null SMTP MAIL FROM is still valid and must be supported (but I admit I haven't kept up with the latest RFCs so I'd appreciate an update if this has changed). It is used in cases where the sender does NOT want to get an error notification (e.g. output of a command to LISTSERV). But it should be owner-xxx on mail from a list iself.