It appears you're trying to setup a new user who has no real login shell. If so then your syntax would be:
useradd -s /bin/false username
The username goes and there should be no space between bin and false - it is a path (do ls -l /bin/false to see it if you want).
Note that the above will use default for home directory and won't create an actual home directory. This also only creates an OS user. It doesn't have anything to do with mail per se. Also since the shell is /bin/false this user would not be able to login directly.
I also always like to add a comment as why the user is there so that when I leave the next guy will. You could do something like:
useradd -c "User added for mail purposes only" -s /bin/false username
Last edited by MensaWater; 10-04-2005 at 01:35 PM.