Hydroxide is a free, third-party and open-source ProtonMail bridge for using
with any email client of your choice. Hydroxide is a server meant to be run
in the background, supporting IMAP and SMTP that translates standard protocols
(SMTP, IMAP, CardDAV) into ProtonMail API requests. Hydroxide stores each of
your account credentials in a json file located in /var/db/hydroxide by default
as encrypted form, then uses .db files per account for other purposes.