A KB article/hotfix has been published about an issue where fax routing to email can fail on Windows Server 2008, upon which SBS 2008 is built. Unfortunately, Windows Server 2008 R2, and therefore SBS 2011 Standard, is also affected by the same issue.
Microsoft explains in the KB973640 article that "consider the following scenario: you install the Fax Server role on a computer running Windows Server 2008 or Windows Server 2008 R2. You configure the Fax Server role to route incoming faxes to an e-mail address. In this scenario, the Fax Service may crash when a fax arrives. Additionally, the sender of the fax receives a notification that the fax is sent successfully. However, the fax isn't routed to the specified e-mail address."
Also, the following events are logged in the Application log:
Cause: The Fax Server role makes some CDO calls to the Inetcomm.dll module to route a fax to an e-mail address. Every time that a fax is routed, the Inetcomm.dll module is loaded and then unloaded. However, two classes aren't unregistered when this module is unloaded. If the module is reloaded at a different address, some WndProc function calls still point to the old address. In this situation, an exception occurs because the old address is invalid. This exception causes the Fax Service to crash.
To resolve this issue, a supported hotfix is available from Microsoft.