Suspended Messages are Included in the Message Count in Database Throttling Threshold
By default the host Message count in DB throttling threshold is set to a value of 50,000, which will trigger a throttling condition under the following circumstances:
The total number of messages published by the host instance to the work, state, and suspended queues of the subscribing hosts exceeds 50,000.
The number of messages in the spool table or the tracking tables exceeds 500,000 messages.
Since suspended messages are included in the Message count in DB calculation, throttling of message publishing can occur even if the BizTalk server is experiencing low or no load.
Recommendations
If you anticipate that you may have a large number of suspended messages, consider increasing the default value for the Message count in DB threshold taking into consideration the space requirements of the SQL server that contains the BizTalk databases. Also consider increasing the Spool multiplier and Tracking data multiplier values to allow additional backlog in the Spool table.
For more information about these values, see How to Modify Resource Based Throttling Settings.
Use the Message publishing throttling state Performance Monitor counter associated with BizTalk:MessageAgent performance object category to measure the current throttling state. If this counter returns a value of 6, then check for suspended instances and terminate or resume suspended instances as needed.
For more information about the host throttling performance counters, see Host Throttling Performance Counters.