If you want archiving, store "plain text" mail on an encrypted disk/along with your other encrypted backup? You will need to archive the public and private key

If you want archiving, store "plain text" mail on an encrypted disk/along with your other encrypted backup? You will need to archive the public and private keys if you need to go back and audit messages - but that assumes that "I verified to my satisfaction in 1996 and archived securely" isn't enough.