I use thunderbird for 3 pop accounts and was starting to get this message on one of them: "Unable to write the email to the mailbox. Make sure the file system allows you write privileges and you have enough space....." Not sure which thing solved it, but I Tried thunderbird in windows safe mode with networking and new emails were downloaded properly with no error message. Then I restarting in normal windows mode and tried Thunderbird again. I got no warning but maybe there was no new emails on that account. Thunderbird asked to compress the files and I said yes. Then I tested all send and receive and all good with no warning.