Dear Igor,
i checked it up the last two weeks (no thunderbird; only the webmailer (roundcube); two different servers with the same bs and version of plesk with courier) - the trash-folder and the spam-folder are drained. Only mails newer than the last 14 days are left in spam-folder. It seems that the mails were deleted on weekend. My guess is, that some kind of a cronjob makes the job. For me this isn't a feature ... I would be glad if you could follow up this behavior.
Thanks in advanced
derde