I recently started using Carbonite to backup my computer on-line. Carbonite marks files for backup whenever they change. The INBOX is constantly changing, and hence will always be marked for backup. (Which will occur within 24 hours of a change). This files can be very large. For example, INBOX for one of my e-mail accounts is over 2 GBytes. I have multiple e-mail accounts, so the total size of changing files (including inbox, sent, and All Mail) is in excess of 10 GBytes. Despite having a fast internet connection (upload speed of ~30 Mps), Carbonite has a transfer speed of about 2 Mbps. (Their explanation is that it does encryption before sending files.) This means that Carbonite will spend at least 11 hours per day backing up these files. Since the problem will grow with time, eventually, Carbonite will be spending 24 hour per day just backing up just mail files. I imagine on-line backup is going to become more prevalent, so this problem is only going to grow and affect more Thunderbird users. I would suggest altering the way incoming mail is handled to use multiple folders, such that the current folder to which mail is added would remain small. I could do this myself by creating local folders periodically that would stay static, but periodic user intervention sort of defeats the purpose of having a backup system that works automatically and in the background. Any other ideas on this? Thanks.