Exchange/Outlook strips ORGANIZER from iMIP replies, causing CalendarImpl.php to discard all incoming acceptances

Hi Everyone, Running Nextcloud 32.0.6 (Hub 25 Autumn) with Mail app 5.7.1, and have fully confirmed a bug where meeting acceptances from Exchange/Outlook 365 attendees are silently discarded — leaving PARTSTAT permanently stuck at NEEDS-ACTION regardless of how many times IMipMessageJob runs. The setup is straightforward: a Nextcloud user creates and sends a calendar invite via the Nextcloud web UI. The external Outlook 365 attendee receives it, accepts it through their native Outlook client, and the acceptance email arrives correctly in the inbox. Everything looks fine — until you realise the CalDAV event never updates. The cause is a two-party problem. Nextcloud generates a fully RFC 5546-compliant invite, including both ORGANIZER and ATTENDEE fields. Exchange/Outlook 365 however strips the ORGANIZER property entirely when generating its METHOD:REPLY, returning only the ATTENDEE field. This is a direct violation of RFC 5546 §3.2.3, which requires ORGANIZER to be present in all re…