Could you copy the exact full error message here or take a screenshot?

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Thank you!

The rectangular box says "Alert! The folder box is full and can't hold any more messages. To make room for more messages delete any old or unwanted mail and compact the folder." I did that and it still comes up with the "Alert!" message.

Have you compacted this folder?

Yes. Did not help.

You can try rebuilding your email indexes, this may also be a message coming from another email account than the one you think.

1. Move any mail in your Inbox - that you want to retain (if any) - to subfolders of Local Folders.
2. Backup your profile. If you cannot see the messages to move them, close Thunderbird and rename the affected inbox file with no extension to something like InboxOld
3. Close down Thunderbird
4. Navigate to your Thunderbird Profile Folder on your system and backup the 'Inbox' and 'Inbox.msf' files elsewhere, then delete them for this account. They will be rebuilt when you relaunch Thunderbird. If you have more than one account, make sure you do this in the proper account. All messages left in the inbox will be deleted in this process. If you do make a mistake, recover from the backups made.
5. InboxOld should show up at this point

The above was adapted from a previous forum post after verifying it.