how to change hostname in sendmail

["Grosswiler Roger" <roger@xxxxxxxx>]

hi,

if i send mails from this host to another, i get rejectes because of the
domainname /hostname. this host is just an internal host and doesn't
figure in any dns. so i send mails in the form host.domain.tld. this form
isn't accepted on my mailserver. how can i change in sendmail form
host.domain.tld to domain.tld?

thx