Do you have any address in the To: some providers refuse BCC: unless there is at least one To: generally the sender inserts their own address as two to get around these provider restrictions.

I think you may have solved my problem! Not certain until I can try another group email, but I really appreciate your help. Many thanks!

In addition to the above, please know that I tried your suggestion but it still didn't work. The most recent information is this: " is not a valid email address because it is not of the form user@host. You must correct it before sending the email" Of course the group name is not a valid email address, only the names and addresses listed within the group are valid email addresses. Why does Thunderbird not understand the difference? Is there a way to make the group name into a user@host?

You might check to see if one of the addresses in the group is malformed.

Is the host named Thunderbird or Mozilla? then maybe the name of the group would be Group@thunderbird.org? Or maybe Group @ mozilla.org? Is that what they are looking for?

There don't seem to be any malformed addresses within the list. It's the name of the list that is not recognized because it's not a user@host. How do I turn the name into a user@host name?

just a trial, add the list to an email and then tab out of the addressing part of the email.

Use you mouse to right click the resultant address pill and select expand from the bottom of the offered menu.

Is there an address in the list highlighted in red? as you see below.