When a Proton user invites another Proton user to a calendar event, Proton resolves the recipient against its internal directory and delivers the invite directly into Proton Calendar — bypassing any external email flow entirely. There is no setting to disable this behavior.

This breaks workflows for users who rely on external calendars (Google Calendar, Outlook) as their primary calendar. The invite never reaches the external calendar, requiring manual .ics export/import every time.

Requested feature:
A toggle under Settings → Calendar → Invitations:

"Receive calendar invitations via standard iCalendar email (disable internal delivery)"

This would restore interoperability with any RFC 5545-compliant calendar client and respect the user's choice of calendar tool — regardless of whether they hold a Proton account.