Email won't transmit

I have some forms designed in LiveCycle Designer 7 that have stopped working. When the user presses the "Submit" button, the dialog box appears, and after selecting the first option, to send the xml data by their default email program, it opens up the email with the fields filled out correctly. But, when they press "Send" nothing happens. This is now happening with me as well, although the forms have not been changed recently and used to work fine.

Any ideas what the problem may be? And/or any suggestions for a fool-proof way to send the data?

Thanks.

Chris