One of the oddities of Thunderbird, compared with other mail programs that I am used to, is that it time stamps the emails using the time they were (apparently) sent according to the clock on the sender’s system. This has some strange effects with mail from people with incorrect clocks, most noticable in spam. Whether it is because they are using hijacked systems to send the mail or it is some devious ploy I don’t know, but I have had mail in the last few weeks with dates from 1 Jan 1970 (just one) through 14 Mar 2000 up until 18 Mar 2007. The old ones go straight from the junk mail box to deleted, but the post-dated ones hang around at the front of the junk box for ages.