Printer system mail messages are coming from user root and get sent to root, as that's the only default member of
the printq group able to run the qdaemon. They also unhelpfully have a blank subject in my experience.
There are three things which may have an effect on the messages:
From "man chvirprt", note the "si" attribute - e.g. for queue "asc" to a remote device "@lpd", to send mail to "nobody":
chvirprt -d @lpd -q asc -a si=nobody
smit chvirprt
- enter queue name from lpstat,
- choose option 2 Default Print Job Attributes
- go to the end ESC->,
------------- Messages/Diagnostics ------------
DIAGNOSTIC LEVEL (normal) - print job; > +
- select "ignore" from the F4 list.
From "man qdaemon", check if you have this file:
/var/spool/lpd/pio/@local/fullmsg # not present by default I think.
However, none of these things stop messages to root on my system 
So the only other approach seems to be some kind of mail filtering,
which I cannot really recommended, and will not by default work with the root user
as sendmail will block it as "unsafe". So tough basically.
Oh, and BTW, to set up to simulate problems, generating messages...
If you have an existing remote print queue, make sure it is "enable"d READY,
unplug the physical printer and print something to it - after a timeout you'll
get the root Message from qdaemon as the queue is taken DOWN when no
contact can be made.
