PGP would not be a good choice for this. The mail archives are still discoverable, and I imagine the decryption keys would be as well. Something closer to OTR

PGP would not be a good choice for this. The mail archives are still discoverable, and I imagine the decryption keys would be as well. Something closer to OTR would be a better choice. Deniability and forward secrecy are the important properties here.