[Bug]: NC doesn't send an email reply to accepted meetings using thunderbird

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I have NC version 25.0.5 with calendar 4.3.4. Using Thunderbird as caldav client of NC.
When i receive an invitation from gmail/Outlook accounts and i accept the meeting using Thunderbird NC creates
the event in my calendar but never send a Reply saying that i accepted/declined the invitation.

If i send an invitation from other NC user using the calendar app and i accept from Thunderbird i get a reply.

The pŕoblem looks like when the organizer is outside the calendar, let's say an external invitation.

Steps to reproduce

1. Create and invite in gmail calendar using as 'Guest' your email associated to NC user.
2. In Thunderbird Accept or Decline the meeting.
3. Check the status of the event in the Calendar web page.
4. NC never sends a Reply

Expected behavior

NC sends a reply accepting or declining the invitation.

Installation method

Community Manual installation with Archive

Nextcloud Server version

25

Operating system

Debian/Ubuntu

PHP engine version

PHP 7.4

Web server

Apache (supported)

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "skeletondirectory": "",
        "defaultapp": "calendar",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "calendar.domain.tld"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "25.0.5.1",
        "overwrite.cli.url": "https:\/\/calendar.domain.tld",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "ldapProviderFactory": "OCA\\User_LDAP\\LDAPProviderFactory",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "25",
        "maintenance": false,
        "session_lifetime": 14400,
        "remember_login_cookie_lifetime": 172800
    }
}

List of activated Apps

Enabled:
  - calendar: 4.3.4
  - circles: 25.0.0
  - cloud_federation_api: 1.8.0
  - contactsinteraction: 1.6.0
  - dav: 1.24.0
  - federatedfilesharing: 1.15.0
  - federation: 1.15.0
  - files: 1.20.1
  - files_rightclick: 1.4.0
  - files_trashbin: 1.15.0
  - files_versions: 1.18.0
  - logreader: 2.10.0
  - lookup_server_connector: 1.13.0
  - nextcloud_announcements: 1.14.0
  - notifications: 2.13.1
  - oauth2: 1.13.0
  - password_policy: 1.15.0
  - privacy: 1.9.0
  - provisioning_api: 1.15.0
  - related_resources: 1.0.4
  - serverinfo: 1.15.0
  - settings: 1.7.0
  - systemtags: 1.15.0
  - theming: 2.0.1
  - twofactor_backupcodes: 1.14.0
  - updatenotification: 1.15.0
  - user_ldap: 1.15.0
  - user_status: 1.5.0
  - viewer: 1.9.0
  - workflowengine: 2.7.0
Disabled:
  - activity: 2.17.0
  - admin_audit
  - bruteforcesettings
  - comments: 1.15.0
  - dashboard: 7.5.0
  - encryption
  - files_external
  - files_pdfviewer: 2.6.0
  - files_sharing: 1.17.0
  - firstrunwizard: 2.14.0
  - photos: 2.0.1
  - recommendations: 1.4.0
  - sharebymail: 1.15.0
  - support: 1.8.0
  - survey_client: 1.13.0
  - suspicious_login
  - text: 3.6.0
  - twofactor_totp
  - weather_status: 1.5.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

No response

Additional info

No response

Start by tracing the Calendar app's CalDAV handling when Thunderbird accepts or declines an invitation from Gmail or Outlook, and compare it with the working flow for invitations from another Nextcloud user. Done means an external organizer receives an acceptance or decline reply, with coverage for both response types.