Invalid DKIM-signature with only BCC-addresses

[sverpoorten]

Hi!

Today I upgraded from version 6.0.1 to 6.6.3 of PHPMailer. This corrected some previous issues, but there is a new one.
The case:

• Using PHP mail() by applying isMail()
• Applying DKIM-signature
• Multiple BCC-addressees
• No To or CC-addressees

This results an invalid DKIM-signature.

What happens is the following:

1. In preSend() line 1607 the To-addressee undisclosed-recipients:; is added to the headers in $this->mailHeader
2. This is used to construct the DKIM-signature (in particular for the z-field)
3. Next in mailSend() the variable $to will be empty since there are no To-addressees and the email will be sent as such

This results in the following example headers (with invalid DKIM-signature):

Received: by mailserver (Postfix, from userid 33) id C53F3520B8D; Sat, 20 Aug 2022 14:00:55 +0000 (UTC)
To:
Subject: Testmail
[...]
DKIM-Signature: v=1; [...] h=Date:From:Reply-To:Message-ID:X-Mailer:MIME-Version:Content-Type:To:Subject; z=Date:Sat,=2020=20Aug=202022=2016:00:55=20+0200 |[...]To:undisclosed-recipients:=3B |Subject:Testmail;[...]

I was able to correct the behaviour by adding the following line after line 1894 in mailSend():
if($to == "") $to = "undisclosed-recipients:;";

This results in the following example headers (with valid DKIM-signature):

Received: by mailserver (Postfix, from userid 33) id CE9AE520036; Sun, 21 Aug 2022 08:45:47 +0000 (UTC)
To: undisclosed-recipients:;
Subject: Testmail
[...]
DKIM-Signature: v=1; [...] h=Date:From:Reply-To:Message-ID:X-Mailer:MIME-Version:Content-Type:To:Subject; z=Date:Sun,=2021=20Aug=202022=2010:45:47=20+0200 |[...]|To:undisclosed-recipients:=3B |Subject:Testmail;[...]

[Synchro]

Good catch. Could you please wrap that up in a PR?

[sverpoorten]

Review-ready :) See PR #2754