“After years of pushing their proprietary and closed solutions to privacy minded people Proton decided that it was in their best interest to further bury said users into their service as a form of vendor lock-in. To achieve this they made more non-standard desktop clients for their groupware features (contacts and calendars) and the bridge will be discontinued soon.”

Only if there wasn’t CardDAV, CalDAV, IMAP, SMTP and dozens of other highly standardized protocols to handle e-mailing and groupware.