The problem here is the Norton product. Nothing you do will modify their death and glory approach to mail destruction, the only really successful approach is to exclude your Thunderbird mail profile from their periodic scans. This works out well as they stop quarantining your old mail because they have an update to their definitions and have now decided the mail you have had for weeks is now a threat. Please also note that the less frequently you allow compaction the more frequently Norton will find an old mail you think you have deleted and do this.