Mailvelope is a free software for end-to-end encryption of email traffic inside of a web browser (Firefox or Chromium) that integrates itself into existing webmail applications (“email websites”). It can be used to encrypt and sign electronic messages, including attached files, without the use of a separate, native email client (like Thunderbird) using the OpenPGP standard.

The name is a portmanteau of the words “mail” and “envelope”. It is published together with its source code under the terms of version 3 of the GNU Affero General Public License (AGPL). The company Mailvelope GmbH runs the development using a public code repository on GitHub. Development is sponsored by the Open Technology Fund.