Thunderbird: Alert! The folder Inbox 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.

The indicated error is displayed at the opening of the Thunderbird mail client when the folder has exceeded its maximum limit.
In this case we must first go to settings - mail account and disable the automatic downloading of email open, close and reopen Thunderbird, and end up leaving the activity "Building summary file for Inbox ..." (visible in the status bar) that given the size of the folder will take several minutes and then select the folder and run the "Compact" in the menu invoked by right mouse button.