The ProtonMail Import-Export app is an application for paid users that
imports emails to your ProtonMail inbox or makes offline backups.

You can import messages from Gmail, Outlook, Yahoo, or any other email
provider that supports the IMAP protocol. Importing from local EML and
MBOX files is also supported. You can use the Import-Export app to
download important messages as MBOX or EML files so that you always have
a backup on your device.

This script repackages the Debian binary provided by Proton
Technologies.
