when I try and send to a group which had worked for years I get: "is not a valid e-mail address because it is not of the form user@host. You must correct it be

When I try and send an email to a group that we have used for years (Image Group) I get the following message upon clicking "send": "Image Group <"Image Group"> <> is not a valid e-mail address because it is not of the form user@host. You must correct it before sending the e-mail." This has never happened before which makes it appear that it is a new Thunderbird issue. We have always created groups/lists as directed and have NEVER seen this message in the past for any group we sent an email to. HELP please. We use Comcast for internet and our address is [email address] Thank you for any help you can provide. Anthony and Lynne Comazzi [phone number]