> If it only happens after checking mail from your proprietary email program
I am not sure whether we can strongly test this against the opposite situation, given the information we have.
The discussion in the link provided by jxself shows that some proprietary mobile device is highly probably causing the email disappearance mystery in the specific case of that specific user. Having an IMAP client set to delete all messages older than 7 days by default is arguably an anti-feature but I would like to understand better what is happening in your case, since you seem to be using IMAP exclusively but not on any tracking device.
I have not found any default setting on the versions you mention that could explain any sort of automatic removal. IMAP clients cannot 'break' the link between the server and the users' machines by default, hence my tentative guess that Thunderbird might not see the messages because the other client might have changed their status from unread to read.
There might me one other case which could explain the situation: if you have created local mailboxes on your old machine for specific incoming email, the content of these mailboxes will not be sync'ed back and the original email will be deleted from the IMAP account and hence from the other machines too once they sync.
