The domain (part after at-symbol) in the sender emailaddress has no MX-record or forward-reverse DNS match.
Sending domain needs MX (not an RFC1918 adresse / local ip)
Connecting IP must have matching forward/reverse DNS.
Important: The sender emailaddress must be able to accept bounces else it might be blocked again.
Questions regarding this error message can be sent to firstname.lastname@example.org (remember to include the error message, ip-address being blocked and your question).