Unable to write the email to the mailbox

A "Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk space to copy the mailbox" error message is typically due to a bug in Thunderbird that can occur if it tries to download a message from a POP server and there is a certain combination of control characters in the message body. This problem only occurs with certain types of POP servers.

You can workaround this bug by reading and deleting the message using webmail (so that Thunderbird won't see it as a new message), and then checking for new mail again. The only way to prevent it from happening is to use a POP server that works better with Thunderbird.