Question : sendmail 8.12.11 Domain of sender address does not exist

I have already installed 8.12.10 on my solaris 2.6
But I have a little problem, I can receive mails from e-mails from my own domain, but when I received mails from an external domain I get this error "Domain of sender address does not exist" on the log.

Dec 27 18:25:08 rubella sendmail[14981]: [ID 801593 mail.notice] iBRNP8L7014981: ruleset=check_mail, arg1=, relay=exchange [172.20.1.25], reject=553 5.1.8 ... Domain of sender address [email protected] does not exist
Dec 27 18:25:08 rubella sendmail[14981]: [ID 801593 mail.info] iBRNP8L7014981: from=, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=exchange [172.20.1.25]

Dec 27 18:30:22 rubella sendmail[16535]: [ID 801593 mail.notice] iBRNUMF9016535: ruleset=check_mail, arg1=, relay=exchange [172.20.1.25], reject=553 5.1.8 ... Domain of sender address [email protected] does not exist
Dec 27 18:30:22 rubella sendmail[16535]: [ID 801593 mail.info] iBRNUMF9016535: from=, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=exchange [172.20.1.25]

Could you help me solving this issue?
Thanks a lot,
Sherly

Answer : sendmail 8.12.11 Domain of sender address does not exist

Sherly,
> My resolv.conf file is like this:
> bash-2.03# more /etc/resolv.conf
> domain rubella.nextel.com.pe
> nameserver 127.0.0.1 172.24.xx.yy

You have your local host set to be your name server and ...

> bash-2.03# nslookup -q-MX yahoo.com
> *** Can't find server name for address 127.0.0.1: No response from server
> *** Default servers are not available

It is not answering name queries.  Remove 127.0.0.1 from /etc/resolv.conf or set up your DNS server to work properly.