Forward all emails from one sendmail to another

- in sendmail.cf change Djdomain.tld to something like Dj_subdomain.domain.tld Do the same thing for DMdomain.tld
- in the same sendmail.cf look for Fw or Fw-o and check the file it refers to (usually /usr/lib/mail/local-host-names); in that file remove the line stating "domain.tld" (in fact, remove everything, leave just _subdomain.domain.tld there), that way sendmail will know that it is not the default destination for the @domain.tld

- if necessary, create /home/$username/.forward (chmod 600, chown $username:$group) and put the right info in it