You may want to change the maximum message size for high priority emails, as the default value is 250KB. The “MaxHighPriorityMessageSize” parameter controls the maximum allowed size for a high priority message.

Navigate to the Bin directory and find EdgeTransport.Exe.Config file. Open it using notepad and find the line that says MaxHighPriorityMessageSize:

 "MaxHighPriorityMessageSize" value="250KB" />

Change the value of “MaxHighPriorityMessageSize” from 250KB to the one of your choice, let say 5MB. You should get something like this:

"MaxHighPriorityMessageSize" value="5MB" />

If a high priority message is larger than the value of the “MaxHighPriorityMessageSize” parameter, the message is automatically downgraded from high priority to normal.