If you do have an email adress ending in @aol.com then email in your folders "New/Read" and "Sent" only stay there for 30 days. If you want to permanently save them, move them to one of the folders under "My folders/Saved on AOL"
Also, if you access mail via POP3, most POP3 clients will delete the copy on the server after they retrieved the mail. If you don't want that, tell the POP3 client to leave a copy on the server.