Sender domain must exist problem

[apichart]

Suppose there are A.com, B.com and C.com
1) In A.com's sendmail.cf, it is configured to relay domain name resolution to B.com as below.
DSB.com
2) When I send the email from A.com to C.com. The C.com domain name would be resolved by B.com since B.com acts as relay server.
3) It is working fine for this.
4) Once C.com send the email to A.com. In mail.log, I found the the error the "Sender domain must exist" How could I define C.com to be known by A.com since When A.com send to C.com we usually use smart relay.

[Chris Saunderson]

http://www.sendmail.org/faq/section3.html#3.22 and also:

http://www.sendmail.org/tips/relaying.html

[apichart]

I don't totally understand the page you kindly provided me. Could anyone tell me easily how to solve this problem? It would be appreciated a lot.

[SELVAM_1]

In c.com sendmail.cf file chk the following line
define(`SMART_HOST', `b.com')dnl

also chk

FEATURE(`accept_unresolvable_domains')dnl
FEATURE(`accept_unqualified_senders')dnl

If not add all three line to sendmail.cf of c.com.