"The header field "To" is syntactically not correct." (554 Transaction failed Reject due to policy restrictions )

Steps to reproduce

I forwarded an email to two recipients (in CC), no entry in To: field.

The @web.de recipient did not receive the message due to "The header field "To" is syntactically not correct." (554 Transaction failed Reject due to policy restrictions ) https://postmaster.web.de/en/case?c=r0735&i=ip&v=85.13.131.209&r=1MbDI4-1v5Tau3J1M-00iYAH

The gmail.com recipient received the message.

Expected behavior

message should be sent & received

Actual behavior

see above

Mail app version

5.1.6

Nextcloud version

"30.0.10"

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

Return-Path: sender@example.com
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=example.com; s=kas202501211002; t=1751790116;
bh=tc0smIGbUSUzylahYO7GPYrY+813l2eF/AlV7xhPdso=;
h=From:To:Cc:Subject:Date:From;
b=t/i0m9vFRz7I7nBLhZp/uW9qpINa/ZUuMWF6oVuMycMD13T3/0Sh3+HdaimJEwQOZ
Wf0xphZ0TFUHWleGeW3eYcbu+pt2yKAZQhJZk858gCp5tR9/HFU8MOFrrq8F0BhSjP
Xs+sqLOmcKTN7B+agvJnA6/evI5x71ciQcWR2r+vGA4YUm1t4dDjVHexvdiJxnrHZO
50swM+Ff5Y3PlXS9lzylqmNAshUSCqIZdQbTNUBydtxAWvIwmZ6ZN220IDTQIpYczp
pAHnIb/rx4hZtMkcJscDuuFcXi9Q+jYh89e348ybPW/uNNyTZJVhXZASb8V5aVYh7i
kD1nEQMAg6L2w==
Received: from example.com [...redacted...]
Sun, 6 Jul 2025 10:21:56 +0200 (CEST)
From: Firstname =?utf-8?b?TcO8bGxlcg==?= sender@example.com
To:
Cc: Recipient One recipient.one@web.de, Recipient Two
recipient.two@gmail.com
Subject: [...redacted...]
Message-ID: [...redacted...]
User-Agent: Horde Application Framework 5
Date: Sun, 06 Jul 2025 08:21:55 +0000
Content-Type: multipart/mixed; boundary="=_MLQt-e4k0YHOv0jzD3zCRuw"
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
X-Spamd-Bar: ---
This message is in MIME format.
--=_MLQt-e4k0YHOv0jzD3zCRuw
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
[Message content redacted]