Aerogramme can't store plaintext data, instead all users data must be encrypted with a per-user key on the storage target. Of course, cryptography is always a tradeoff with other properties (usability, compatibility, features, etc.), so the way the key is derived and where the encryption/decryption can take place can be configured.
Compared to PGP
PGP only encrypts the body of the email, it keeps in cleartext the metadata of your email (fields like From:, To:, or Subject: are readable by an attacker), it can't protect your flags, your mailbox names, etc. Conversely, all this data is encrypted in Aerogramme.
These different configurations are identified as flavors:
Transparent Flavor - Users' data are decrypted in the server's RAM. The key is derived from the user's password upon IMAP login. It's completely transparent to end users. While an attacker having full access to the server can still compromise users' data, it reduces the attack surface. In term of security, it's similar to TREES (from RiseUp) or scrambler (from Posteo).
Hardened Flavor - Users' data are decrypted on the user's device. The private key is only stored on the user's device. The server knows only users' public keys. When an email is received by the server, it is directly encrypted and stored by the server. The user must run Aerogramme locally as a proxy that will manipulate the encrypted blobs stored in the server and decrypt them locally, exposing an IMAP proxy interface. An attacker having full access to the server at a point in time will not be able to compromise your already received data (but can intercept new emails). It's similar to Proton Mail Bridge, but keep in mind that Aerogramme does not support (yet) end-to-end email encryption like Proton Mail or Tutanota, so Aerogramme is less secure.
The transparent flavor only requires Aerogramme to be run on the service provider server, while the hardened flavor require the end-user to run a local proxy. More specifically:
Provider - Provider must be run by the service provider, it is used for both flavors. For the transparent flavor, it both receives emails through LMTP and expose
the mailbox through IMAP. For the hardened mode, it only receives emails through LMTP, encrypt them with user's public key, but can't expose them through IMAP as the server
can't decrypt them. Provider commands are available through the
aerogramme provider subcommand.
Companion - Companion must be run by the end user, it is used only for the hardened flavor. It fetches encrypted blobs from the server
of the email provider, decrypt them locally, and expose the mailbox across the IMAP interface, acting as a local proxy.
Companion commands are avaialble through the
aerogramme companion subcommand.