I've recently set up dns on this linux box and now when I try to send regular mail I get it sent back with the error that sender domain must exist. Since my domain is no longer localhost@localdomain I think there is now something missing from sendmail. I've looked in the sendmail.cf file and have read a few posts on other newsgroups but its rather confusing. Any advice would be appreciated. I'm running red hat 7.

What version of sendmail are you running? Have you put the name of the machine in sendmails list of local host names?

I have sendmail-8.11. I put 'linuxhost.mydomain.com' in local-host-names but it made no difference.