Far be it for me to be negative, but I have been chasing a small problem that has suddenly appeared with my Turnpike email client and AVG antivirus.
In the process, I have visited the AVG forums and read the following about the total loss of emails in Thunderbird
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one follows the links through, the message seems to be that this is a problem with compacting Thunderbird files, and the main cure seems to be "Try the beta of the next version (16) to see if that cures it".
I only use TB for gmail, so I'm probably oK, but it all looks a bit worrying to me.