Over the past week I have encountered a problem in sending e-mail to groups in my address book. I am able to do so and then for some unknown reason I no longer am able to do so.  The condition comes and goes. When attempting to Send, I get a pop-up window stating:

"This message could not be sent

. Type an e-mail address for one or more recipients in the

To; Cc, or the Bcc box.

Can someone please tell me how to fix this problem?  Thanks for your help.