That's a Mailing List, not an address book.

I suspect that the host* address book is corrupted. The simplest way to deal with this is probably to re-create the Mailing List. You can probably do a bulk drag and drop to copy the contents of the current Mailing List to a new one. You can populate a Mailing List by dropping Contacts onto its icon.

An alternative is to export the host address book to an LDIF file, delete or rename the original address book then import the LDIF file to create a new copy of the original address book.

[*]When I say "host" I mean that the Mailing List will appear as if it is a subfolder inside another address book. It's that address book that is damaged.

PS. I do hope you are using Bcc: with your Mailing List. If you use To: each recipient will see all the other recipients' addresses!

Thanks Zenos. Now I'm even more confused. On my Thunderbird program, my email addresses are stored in something called "Address Book" and within the Address Book, there are sub folders in something called "Personal Address Book." Are you saying that within the Personal Address Book, there are different "kinds" of sub folders. Some folders are just static repositories of email addresses, while other sub-folders are dedicated "mailing lists"? How does one tell the difference? They all look the same, with no defining characteristic (??) Isn't there just some fast method to say "make all my folders usable as mailing lists?"

I'm good to try re-creating the "mailing list" but I have no idea what that is, or where to find it. When I explore Thunderbird / Profiles, I see a file called abook.mab, but no way to drill down into it. I tried exporting the desired folder as a CSV file and then reimporting into a new file, but no luck.

In the Address Book section, I discovered that when I right-click on the root name of the Personal Address Book, I can navigate into More Functions / Recover-Restore. Is this what you meant by "re-create the Mailing List?"

The terminology is unfortunate and confusing. Thunderbird has an Address Book. It contains two default subdivisions, also referred to as "address books", named Personal Address Book and Collected Addresses. These two are provided by default and will be regenerated if you try to delete them.

You can also add further address books which will be at the same level as the two address books named above.

Inside any address book (e.g. Personal Address Book) you can create Mailing Lists, which are presented as if they were sub-folders. A Mailing List is just a collection of Contacts from its parent address book. You can't treat a Mailing List as a subfolder of its address book; well you can, but what happens is that if you try to add a new (previously unknown) Contact into a Mailing List, a new Contact is created automatically in the parent address book. A Mailing List is merely a sub-set of Contacts taken from its parent or host address book. It is not a self-contained address book.

Many users refer to Mailing Lists as "Groups" or "Distribution Lists", presumably borrowing terms from other email clients. It really does help if people actually quote what they see in front of them rather than inventing their own names. ;-)

The whole point of Mailing Lists is that, as you have found, you can use the Mailing List's name in place of a real email address, and it represents all of its member Contacts. (Until, of course, it is broken and Thunderbird stops recognizing it as a Mailing List.)

Now I have pointed you to the appropriate terminology, you should be able to search for "Mailing List Thunderbird" and find further advice on their use and maintenance.

I guided you to use LDIF because it preserves Mailing Lists. CSV are simple "flat" data files and can't store nested structures - at least not in a way that would be portable and meaningful in all the different contexts in which a CSV file might be used.