What is the "from" address of mail A, the one that succeeds?
It is probable that:
- your sendmail is configured so it does not do any DNS lookups and simply sends all mails to the relay host 172.30.2.13.
- the relay host is also a DNS server.
- the relay host is configured to use itself for DNS.
You could verify item one by comparing your sendmail configuration to the one described in the page linked in this post