Mail unsubscribe fails although server returns HTTP 200 OK

Steps to reproduce

1. Open a newsletter email in the Nextcloud Mail app.
2. Click the “Unsubscribe” button in the top-right corner.
3. The email contains a valid List-Unsubscribe header with an HTTPS URL, for example:
   List-Unsubscribe: https://www.abcde.com/unsubscribe-contributor.php?...
4. The server responds with a standard HTTP 200 OK and a confirmation page.

Expected behavior

Nextcloud Mail should treat a successful HTTP 200 OK response as a valid unsubscribe confirmation (or at least not show an error message).

Actual behavior

Nextcloud shows an error notification:
“Unsubscribing from the mailing list failed”
even though the unsubscribe request was successfully sent and the server returned 200 OK.

Mail app version

• mail: 5.5.11 - sharebymail: 1.22.0

Nextcloud version

32.0.1.2

Mailserver or service

GMAIL, GMX

Operating system

Debian 12 Proxmox 8.4.14 LCX

PHP engine version

PHP 8.2

Nextcloud memory caching

\OC\Memcache\APCu | \OC\Memcache\Redis

Web server

Nginx

Database

PostgreSQL

Additional info

Environment

Component	Version / Info
Nextcloud	30.0.1
Mail app	3.7.0
Operating system	Debian 12 (LXC on Proxmox)
PHP	8.2.16 (FPM)
Web server	nginx 1.24
Database	MariaDB 10.11
Mail service	smtp.gmail.com (587 / STARTTLS)
Memory cache	Redis
Container type	unprivileged LXC
Networking	IPv4 only (IPv6 disabled)

Additional info

• Outbound HTTPS confirmed working (curl returns 200 OK)
• Browser unsubscribe link works correctly
• Nextcloud Mail unsubscribe fails with “Unsubscribing failed” although server returns HTTP 200 OK.

No file or test is named. Start by tracing the Mail app's unsubscribe action from the top-right button through its HTTP response handling, then reproduce the HTTPS 200 OK case and inspect why it reaches the failure notification. Done means a valid HTTP 200 response is treated as successful without showing the error.