mail queue problem

I have created a website which sends out automated emails to users
subscribed to the website using the mail_queue package.  The mails that
these users receive are not intended to be replied to so the email address
should not be displayed in the email.  However, every time I try to set the
$from variable to some name like "system administrator" the mail fails to
send.  If I set $from to empty it also fails to send.  Is there any way I
can stop the email address from showing up in an email and replacing it with
just some generic name instead.  In theory for a mail to be sent
successfully all you need to provide is a correct recipient address anyway.

Any help would be appreciated, thanks!