Email body displays email header information due to blank line

I am using 1.3.9h, email setting is text only, smtp and the encoding that I am using is Japanese encoding. All of the emails that get created by zenCart has some part of email headers displayed in the email body in error.

It seems like the header is chopped because there's an extra line feed within the header text.

Anyone knows how to fix this? I took a look at the function_email.php and mail.php(admin) but I don;t see anywhere that constructs the X-Nonspam and Message ID. How can I debug this?

FYI: This is the email source, see there's a line between "X-Nonspam: None' and the 'Message-ID' lines?

Subject: =?ISO-2022-JP?B?ZW5nbGlzaA==?=
X-Nonspam: None

Message-ID: [email protected]
X-Priority: 3
X-Mailer: PHPMailer [version 1.73] via Zen Cart
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset="ISO-2022-JP"