DKIM - Header 'Subject' is not signed

[novami]

Error "Header 'Subject' is not signed" in some 3rd party DKIM validation tools (like extension for Thunderbird).

See lines below to understand:

mail/src/Mail/SendmailMailer.php

Line 42 in 8bb8926

$tmp->setHeader('Subject', null);

mail/src/Mail/DkimSigner.php

Line 21 in 8bb8926

'Subject',

mail/src/Mail/DkimSigner.php

Line 148 in 8bb8926

return array_filter($this->signHeaders, fn($name) => $message->getHeader($name) !== null);