Empty "To" field in header with only bcc recipients

[xeiss]

Steps to reproduce

1. Write an email to a GMX user with only using bcc recipients, no one in "to"
2. Get message from GMX, that this email was rejected:
   554-Transaction failed 554-Reject due to policy restrictions. 554 For explanation visit
   https://postmaster.gmx.net/en/case?c=r0735
   -> "The header field "To" is syntactically not correct."
3. Look in email attached:
   From: Name e-mail@example.com
   To:
   Subject: Test
   -> "To:" is not correct because it is empty

Expected behavior

When you send an email to only bcc recipients, no "To:" field in header

Actual behavior

Empty "To:" field is not allowed in rfc5322 (address-list is not optional when header is used)

Mail app version

5.11.5

Nextcloud version

34.0.3

Mailserver or service

GMX

Operating system

Windows

PHP engine version

PHP 8.3

Nextcloud memory caching

No response

Web server

Apache (supported)

Database

MariaDB

Additional info

No response