Technically No. Practically 4Gb is still the limit I would consider as the removal of the limit is to new to be trustworthy. * Loss of mail in the mailbox can be caused by a corrupt index (right click the folder select properties and then repair) * Quarantined by anti virus (look in the anti virus logs for a file called inbox) * Hard disk errors (check disk with disk tools that come with the operating system) * write errors caused by other programs (anti virus) locking the file to scan it (Create an exemption for the Thunderbird profile folder in your anti virus programs on access scanner) Further information here http://kb.mozillazine.org/Disappearing_mail