Home Page: https://mailvelope.com
Zimbra: https://zimbra.virtualarchitects.com/service/zimlet/tk_barrydegraaff_zimbra_openpgp/help/index.html
RoundCube: Mailvelope Enigma Plugin
Mailvelope is a tool to send and receive encrypted messages and files to other users. It uses PGP public key cryptography with a default RSA private key size of 4096 bytes, widely considered to be uncrackable.
Mailvelope is primarily a tool for encrypted e-mail, but it can easily be used to encrypt or decrypt files as well.
Mailvelope already works with many webmail providers:
https://mailvelope.com/en/faq#mailer_list
Mailvelope is a browser add-on that you can use in:
Versions: https://mailvelope.com/en/products
Wiki: https://github.com/mailvelope/mailvelope/wiki
Tutorial: https://mailvelope.com/en/help
Generate a public and private key pair first. Export both keys and back up the .asc file.