You noticed multiple similar messages in the following log output in syslog on the SMS-FW system and are wondering what this means.
textpass: MT queue holds 51 messages, check the recipient variation of your test scenario
This is an informational syslog. There is no impact on how RTR behaves. The message is created when there are more than 50 messages to the same recipient, waiting to be delivered in RTR. The 50 message limit is hardcoded to the RTR and is not configurable.