Check Mail flow queue Back Pressure

 

Exchange Shell - 

[xml]$bp=Get-ExchangeDiagnosticInfo [-Server servername01 ] -Process EdgeTransport -Component ResourceThrottling; $bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter

[xml]$bp=Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling; $bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter