Exchange 2016 Back Pressure

This morning I wasnt recieving any external emails so I check my exchange 2016 box and notice this error

Event 15006, MSExchangeTransport


Microsoft Exchange Transport is rejecting message submissions because the available disk space has dropped below the configured threshold.The following resources are under pressure:
Used disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue")
Used disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data")
Overall Resources

The following components are disabled due to back pressure:
Mail resubmission from the Message Resubmission component.
Mail submission from Pickup directory
Mail submission from Replay directory
Mail resubmission from the Shadow Redundancy Component
Inbound mail submission from the Internet

The following resources are in normal state:
Private bytes
Queue length[Submission]
System memory
Version buckets[D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que]
Queue database and disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que")

My D: drive is 602GB and I have 96GB free so I'm sure why this was back pressure was triggered or I'm just not understanding the formula.

I plan on adding addition disk space to this drive but I would like to understand what the threshold is so I dont run into this problem again.  For the moment I have it disable through the EdgeTransport.exe.config.  

Can someone tell how I can determine what the threshold for disk space?