If your e-mail INBOX gets too large, the system may automatically archive some of your older e-mails to a folder in your account.
If you suddenly notice all your old e-mail from before some date is no longer in your INBOX, then it was probably archived.
Archived mail is placed into an e-mail folder in your account called "old-mail-before-XXXX.gz" where XXXX is a year (e.g., 2011). This folder is gzip compressed to save disk space. Such compressed folders may still be read by your normal e-mail clients, but can not be changed (they are considered "read-only" -- to make changes, you have to decompress them first).