When attempting to deliver messages outbound to the internet, some messages becomes stuck in the delivery queue with a "no MX or A for domain" message even though a lookup of the domain's MX records returns valid mx hosts. Looking up the DNS records shows something similar to the following:
domain.org. 1951 IN DNAME domain.com.
mx.domain.org. 1951 IN CNAME mx.domain.com.
mx.domain.com. 1951 IN MX 10 smg3.domain.com.
mx.domain.com. 1951 IN MX 10 smg1.domain.com.
mx.domain.com. 1951 IN MX 10 smg2.domain.com.
454 4.4.4 [internal] no MX or A for domain
2014 Jul 29 09:42:24 PDT (debug) ecelerity:  LT-00284 inserted 80/00-08666-0FEC7D35 into lookuptable (calling dns_get_MXs)
2014 Jul 29 09:42:24 PDT (debug) ecelerity:  ARES_00320 process dns response 17465 [mx.domain.org]
2014 Jul 29 09:42:24 PDT (info) ecelerity:  smsrouting: DNS reply to MX lookup of \'mx.domain.org\': wrong answer name \'domain.org\'; unroutable messages may be stuck in domain container \'#sms#00000005\'
Messaging Gateway 10.6
Some DNS servers return the DNAME record as part of the response to the MX record lookup. The SMG mail server does not understand the DNAME record and so does not process the DNS response.