Exchange 2007 or SBS 2008 suddenly stops receiving external emails.
Internal and outgoing emails are working fine. Emails are not bouncing back. Exchange Management Console seems to be OK. Exchange stores mounted and all Exchange services running.

This behaviour could be caused by a low space on one of the hard drives. Disk doesn’t have to be completely full, just getting close to this.
Check Application logs for following events:

Event ID: 15004
Level: Warning
Source: MSExchangeTransport
Resource pressure increased from Medium to High.
Resource utilization of the following resources exceed the normal level:
…..

Event ID: 15006
Level: Error
Source: MSExchangeTransport
The Microsoft Exchange Transport service is rejecting message submissions because the available disk space has dropped below the configured threshold.
Resource utilization of the following resources exceed the normal level:
…

Simply free up some space on the hard drive in question and email delivery will resume. On Small Business Server 2008 the first suspect would be WSUS database.

Windows Small Business Server 2008
Exchange 2007