subdomain

hi all,
i am trying to set up a mail subdomain redirect.foo.com on a server
that now hosts the mail domain mail.foo.com
i have already added the cname record redirect.foo.com that points to
the same ip of mail.foo.com and it works
my problem is that, when i try to send a message to
name@redirect.foo.com, i get a "user unknown" error
the things i tried:
- added the subdomain in local-hosts-names
- added the line for name@redirect.foo.com in virtusertable
- edited the sendmail.mc file adding the following lines:
FEATURE(`accept_unresolvable_domains')dnl
LOCAL_DOMAIN(`redirect.p3italy.it')dnl

what more should i do?

tia
lorenzo