[Exim] Is it an Exim configuration error?

When I send mail to any mailbox handled by FreeBSD.org, it returns with
a strange error message such as:

  ports@???
    SMTP error from remote mailer after RCPT TO:<ports@???>:
    host hub.freebsd.org [216.136.204.18]: 450 Client host rejected:
    cannot find your hostname, [202.106.127.54]: retry timeout exceeded

But I have never met such a problem before I changed email server to
use Exim as MTA, which used to be running on qmail.

Thanks a lot!

--
Levin Lee <llee@???>
Consultant
LDCONFIG SOFTWARE