Starting from Exchange 2007,  the edge transport service uses the system resource monitoring feauture to determine vital resources such as hard drive space and memory and take action in an attempt to prevent service outages. This can be a pain as the whole thing is calculated on a formula,

100 * (hard disk size – fixed constant) / hard disk size

this feature is called Backup pressure, more info on http://technet.microsoft.com/en-us/library/bb201658.aspx and how this formula works http://www.msexchange.org/articles_tutorials/exchange-server-2010/management-administration/back-pressure-exchange-2010-part1.html

Now simply put, you have this error from other smtp servers trying to contact yours : Insufficient System resources, to get around this you will need to increase the hard drive space or memory, until then you could temporarly disable this monitoring feature so that you can start recieving important emails. WARNING: this is not a recommended permanent solution, do this until youve figured what resource needs looking at i.e hard drive space, RAM etc.

Make a backup copy of the edgetransport.exe.config file before you start.

1. Open the EdgeTransport.exe.config file from \Exchange Server\bin directory using notepad
2. Add the following key+value pair:
   <add key=”EnableResourceMonitoring” value=”false” />
3. Save file
4. Restart the Microsoft Exchange Transport Service (MSExchangeTransport):
   Restart-Service MSExchangeTransport
5. and you’ve got mail

Other options are to

configure the edgetransport.exe config file to reflect your server configuration and resources (not recommended by MS)

OR

move the queue database to another volume which has lots of space, add the following key to change the path of the queudb.

1. Open the following file by using Notepad: C:\Program Files\Microsoft\Exchange Server\V14\Bin\EdgeTransport.exe.config.
2. Modify the following line in the <appSettings> section.

   <add key="QueueDatabasePath" value="<LocalPath>" />

   This example creates a queue database at the location C:\Queue\QueueDB.

   <add key="QueueDatabasePath" value="C:\Queue\QueueDB" />

3. Save and close the EdgeTransport.exe.config file.
4. Restart the Microsoft Exchange Transport service.
5. Verify that the new Mail.que and Trn.chk files are created at the new location.
6. Remove the unused Mail.que and Trn.chk files from the original location.

ref: http://technet.microsoft.com/en-us/library/f170cb0c-04a9-4fa7-b594-206e3a787e14.aspx
The following event logs will be logged:

• Event ID 15004: Increase in the utilization level for any resource (eg from Normal to Medium)
• Event ID 15005: Decrease in the utilization level for any resource (eg from High to Medium)
• Event ID 15006: High utilization for disk space (ie critically low free disk space)
• Event ID 15007: High utilization for memory (ie critically low available memory)

Have look out for the Event id which will help you in diagnose what resource needs to be looked at.

ref:http://exchangeserverpro.com/exchange-transport-server-back-pressure