Messages accumulate in the delivery queue with the following queue status:
421 4.4.4 [internal] Domain Does Not Resolve
Looking up the MX records for the domain returns results as expected but the results are of mixed or upper case.
The MTA log shows
(info) ecelerity:  smsrouting: DNS reply to MX lookup of 'domain.example': wrong answer name 'DOMAIN.EXAMPLE'; unroutable messages may be stuck in domain container '#sms#00000042'
DNS provider returns upper case MX records for multiple different domains and domain name comparison is case sensitive on DNS results.
This has been addressed as of Patch 291 for Messaging Gateway version 10.7.5.
For Messaging Gateway patch information please refer to the following article, which includes how to view patch notes and how to install patches:
For versions previous to 10.7.5 with patch 291, a potential workaround is to switch to a DNS server that returns results in lower case.