[Bug]: External attendee invitation marked failed (schedule-status 3.7) while SMTP test succeeds on Nextcloud 34.0.5 #65882

Bug description

When an external email attendee is added to an event using the Nextcloud Calendar web UI, the attendee shows "Failed to deliver invitation". The stored iCalendar attendee entry for the same external recipient (in another event) contains SCHEDULE-STATUS=3.7.

This is unexpected because the administration UI has Send invitations to attendees enabled, and a test email sent from the administration SMTP settings succeeds.

Important limitation: The SCHEDULE-STATUS=3.7 value was confirmed in a different event with the same external attendee; the iCalendar record of the originally reported event was not extracted. This issue reports a suspected invitation-delivery problem, not a confirmed root cause.

Environment

• Nextcloud Hub 26 Spring, version 34.0.5
• Calendar web UI (Calendar app version not yet established)
• Self-hosted Nextcloud
• Admin > Groupware > Calendar server: "Send invitations to attendees" enabled
• Admin > Basic settings > Email server: SMTP, port 587; Send test email succeeded
• External attendee email address omitted for privacy

Steps to reproduce / observed behavior

1. Create or edit a calendar event in the Calendar web UI and add an external email attendee.
2. Save the event.
3. The external attendee is displayed as "Failed to deliver invitation".
4. Independently, a database inspection of another event involving the same attendee shows an ATTENDEE property with PARTSTAT=NEEDS-ACTION;SCHEDULE-STATUS=3.7.
5. An SMTP test email sent from Nextcloud's admin settings reports success.

Expected behavior

An email invitation should be delivered to a valid external email attendee, or the Calendar UI should provide an actionable reason for delivery failure.

Actual behavior

The Calendar UI reports failed invitation delivery despite an apparently operational SMTP test path. The underlying reason remains unconfirmed.

Diagnostics

• ATTENDEE;...;SCHEDULE-STATUS=3.7:mailto:<external-address> (obtained from one calendar object; recipient redacted).
• Recent Nextcloud log searches did not expose a clearly matching invitation-delivery exception.
• No configuration changes were made during investigation.
• RFC 6638 uses 3.7 for invalid calendar user, but that alone does not explain whether external email delivery was attempted or should have been.

Please advise which additional non-sensitive evidence would distinguish an expected scheduling-address validation outcome from an iMIP delivery failure.