If you get this error when you are trying to send email to Yahoo. It is very likely that your server ip is black-listed.
You can contact yahoo postmaster by filling in the form here: http://help.yahoo.com/l/us/yahoo/mail/postmaster/errors/?referrer=blog.my-helper.com
Another easier solution is to change your server IP.
The problem could be solved by:
- Adding Message-ID to the header
- Encoding the subject in base64 form:
$subject = ‘=?utf-8?B?’ . base64_encode(‘EMAIL SUBJECT’) . ‘?=’;
- Using php mail() function instead of PHPMailer