Dejanews is rather specialised in it, but other engines can search Usenet as well. But certainly not for all. Every e-mail you sent is potentialy everlasting. I don't suppose somebody is tracking down all your mails, but I don't think it's impossible to do so, if one really tried. Since most mail messages consist of text only, they are easily read once found.
Encryption can be an answer.
First of all a webmaster can keep simple statistics . Like the counters you often see, or the NedStat statistics I use. With them, he knows only little about you. And everything he knows, you can find out as well, since the results are published on public pages. Click this image for access .