Reason
Your email bounced due to an incorrect HELO response: either your server is using various different HELOs or the reverse HELO lookup failed.
Solution
Make sure that your email server does not send an alternating HELO. In addition, the HELO/EHLO must comply with the requirements in RFC RFC 5321. You can check your HELO e.g. here.
See also: General information on our error messages.