The ProtonMail Bridge is an application that runs on your computer in the background and seamlessly encrypts and decrypts your mail as it enters and leaves your computer. It allows for full integration of your ProtonMail account with most clients that support IMAP and SMTP protocols, with official support for Microsoft Outlook, Mozilla Thunderbird and Apple Mail. This includes support for importing emails into your ProtonMail account from other email accounts that are connected to the same client, as well as full-text search of your ProtonMail emails from within your client.
Currently, versions for macOS and Windows are available at the ProtonMail Bridge homepage, along with frequently asked questions and detailed setup instructions for each of the supported clients. The Bridge release for Linux, which will include a command-line interface version, is scheduled for early 2018.
The ProtonMail Bridge software is currently only available for paid ProtonMail users, although this may change in the future if we have the infrastructure to support free users as well.
We do not have POP3 support at this time, but the Bridge will allow you to download all of your mail into your client and export it from there.