The full error message I'm receiving is "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." I'm using pop mail for this account. The server is Yahoo. I've tried the obvious fixes: I've compacted the inbox folder and cleaned up folders on the server. My inbox file is only 536 MB on my local machine and on the server I just moved more than half the mail from the inbox to a separate folder. I don't want to switch to IMAP for this account because there are years of older mail in separate folders on the server which I don't want to download again. I do keep older mail around, but it's in separate folders by date. Is there an upper limit to the number of folders that Thunderbird can support, or maybe a limit to the total amount of data that can be kept in all folders? I have read through a number of dialogues on Mozilla support but nothing seems to work for this situation. Are there other fixes available when receiving warnings like that? Thank you!