This solution is only part of the fix, you must also plan ahead to delete or move files out of this drive so it doesn’t run out of space and completely corrupt and destroy the server. This fix will just make mail start working in the meantime.
While testing email, I looked in our exchange queue and got message “452 4.3.1 insufficient system resources” while trying to email the server with the issue. I believe it will also have various errors in the event viewer about space and exchange.
To force mail to start flowing, do the following: