Daylight savings time
From MozillaZine Knowledge Base
A bug in Mozilla Mail and Thunderbird can cause a variety of problems to occur when your computer’s clock changes due to daylight savings time (DST) or switching time zones. You may be experiencing this bug if you see any of these problems:
Metadata about your mail and your mail folders are stored in mail summary files (*.msf) in your profile folder. When your computer’s clock changes, some of the timestamps stored in the summary files may differ from the files’ timestamp. This causes all the data in the file to be considered invalid, and the loss of data manifests itself as the problems listed above.
Who is affected
This bug currently only affects profiles stored on FAT32 partitions (or file shares). In older versions of Thunderbird, other filesystems were affected as well.
Exit Thunderbird and delete all of the *.msf files for every account in your profile, including any accounts that seem to be unaffected. Thunderbird will build new *.msf files the next time you run it.
Note: Your messages are stored in text files with the folders name and no file extension, not in the *.msf files. For example, the messages for your inbox folder are stored in “inbox.”, not “inbox.msf”. You can verify this by opening the file with a text editor.
You may want to backup your profile beforehand in case you make a mistake.
If you have problems with just your inbox folder afterwards, it may have been slightly corrupted beforehand. Try replacing the inbox folder.
Set the preference mail.db_timestamp_leeway to a number greater than 3600 to allow Thunderbird to ignore variations in *.msf file timestamps caused by DST. This has been changed for Thunderbird 2.0. You can add it in 1.5 by:
Compact all of your folders beforehand. This will physically delete any deleted messages, which are actually just marked for deletion and hidden until you compact the folder. That should prevent deleted messages from being resurrected and reduce the chance of a folder getting corrupted.
The “Keep it working” article describes several things you can do to minimize problems in general, such as automatically compacting your folders.