The insufficient resources messages is to do with a test that the transport service performs on disk space. The system checks that 15% of the system disk is free and will not allow incoming mail until this is resolved.
You can circumvent this with the following:
Browse to C:\Program Files\Microsoft\Exchange Server\V#\bin and open edgetransport.exe.config with notepad.
Press CTRL + F and search for the term EnableResourcesMonitoring
Change the True to False.
Save the file
Restart the transport service
Very often the IIS logs can fill up if you are logging issues and take up most of the C Drive on an Exchange 2016/13 box.