I have Mozilla Thunderbird and I find that some emails particularly with attachments (but not always) disappear from my inbox after a few months and I can't find them by doing a search under the sender's name or in my inbox. As an example my most important emails come from 2 sources. One is an accountancy practice and the other a property management company. Those from the accountancy practice are nearly always missing after a few months but those from the management company rarely are. If I carry out a search on the accountancy practice name, nearly all of the emails in the list are ones that I have sent and the only way I can find a particular email is to look at the trail before my email, but by doing so there are no attachments with their email. If I then look at the date of their email in the trail and then search for the date in my inbox, the email isn't there.