I'm  currently  trying to collect old data, most importantly e-mails I
  sent and received in the 90s. I even found about 350 mails  from  back
  then. Luckily, they were stored in mbox[1] files, so I could easily[2]
  import them into my current maildir[3] structure.

  See, they *could have been*  stored  in  some  proprietary  or  binary
  format.  But they weren't. I used Netscape Mail back then which stored
  everything as mbox. I guess, Thunderbird still does  the  same  today?
  Dunno.

  It  is vital that we save important data in simple file formats. It is
  vital that we use free and open file systems. Otherwise, our data  can
  be considered lost.

  ____________________

  1. https://secure.wikimedia.org/wikipedia/en/wiki/Mbox

  2. http://www.uninformativ.de/?section=news&ndo=single&newsid=107
     (german)

  3. https://secure.wikimedia.org/wikipedia/en/wiki/Maildir