Sendmail to username without FQDN

I have some Solaris 10 machines that cannot send an email without @domain added. For instance, myusername@host.com will work, but just using myusername will not. I also have some Solaris 8 machines, but I can send emails with or without the @domain added. Does anyone know what is making sendmail on sun 10 not able to send without the @domain portion of the address? Thanks!