unresolvable sender domain in sendmail

Dear all,

Under sendmail server in redhat 9, I find that emails from one of the real
domain are sent to any users of the sendmail server and the error occurs,
i.e. Unresolved sender domains.
Anyway, such domain is real but not unresloved.
I think the occurrence of this error is due to the statement
"FEATURE(`accept_unresolvable_domains)" in the config file "sendmail.mc".
Anyway, commenting this statement is to prevent spam emails from sending to
the email server.
Welcome for any of your advice, many thanks.

Regards,
John