You get the the following error in mymailsecurity:
Send mail through your approved SMTP relay, or add a PTR record so the rDNS check more closely resembles their HELO name.
This error also occurs when there is an SPF record mismatch and the receiving server has a robust SPF checking rule.
Applies to the following Optimus Systems product
mymailsecurity Email Security
Keep your SPF record up to date
Make sure that your SPF record is up to date and includes mymailsecurity references.