The same issue happens in MS Excel 2007. The issue is only ocurring on some user machines, not all for our 100 person company.

Outlook is the default email client. The error message: The messaging interface has returned an unknown error. If the problem persists, restart Outlook.

This issue occurs with .doc extensions but does not occur with .docx extensions. Management wants to be able to send files to our core customer base of small businesses without having to save as .docx or close a file and attach starting from Outlook (which works for .doc files, but is not as productive as sending a file already open on the screen).

Turning off Marcro security creates more error messages about MAPI - seeming to indicate that Word/Excel cannot tell there is a default email client installed.

I have tried creating a new user profile in Outlook and uninstalling/reinstalling. I have tried changing many settings in Word. Please post a proven and tested solution if you have one. Also interested in knowing if anyone has found success when changing the Outlook Trust Center setting to enable "Add properties to attachments to enable Reply with Changes". Thanks!