SMTP error 550 due to multiple In-Reply-To headers in a single email

Acording to my host Siteground, there are script errors that cause smtp error when mailing contacts. This is vTiger 7.01.

Below is the message I've received from Siteground techs:

I have carefully investigated the cause for the issue and it appears that the scripts modules/Emails/models/Record.php and modules/Emails/mail.php contain code which stacks multiple In-Reply-To headers in a single email, which is being rejected by the mail server. In order to make the emails sendable, I had to manually disable the following code in both files:

Code: if (!empty($inReplyToMessageId)) { $mail->AddCustomHeader("In-Reply-To", $inReplyToMessageId); }