Error message when you use OWA to open a signed e-mail message that is sent as an attachment to an unsigned e-mail message: "The message has a digital signature, but it was not validated"
To resolve this problem, install this hotfix. After you install this hotfix, you must enable it by adding a CheckEmbeddedSmime registry entry to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeWEB\OWA registry subkey. To do this, follow these steps:
1. Click Start, click Run, type Regedit, and then click OK.
2. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeWEB\OWA
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type CheckEmbeddedSmime, and then press ENTER.
5. Right-click CheckEmbeddedSmime, and then click Modify.
6. In the Edit DWORD Value dialog box, under Base, click Decimal.
7. In the Value data box, type 1, and then click OK.
8. Exit Registry Editor.
http://support.microsoft.com/default.aspx/kb/937851