Emails from our mail server gets rejected

Hi good day, i would like to ask how come the emails the we send from our systems gets rejected from our clients using KPN as their mail service. 

reject message as shown below

 

<client>.kpnxchange.com rejected your message to the following email addresses:

<recipient_email>
A problem occurred while delivering your message to this email address. Try sending your message again.
 

<client>.kpnxchange.com gave this error:
Mail rejected.

Diagnostic information for administrators:

Generating server: VE1PR10MB3376.EURPRD10.PROD.OUTLOOK.COM

<recipient_email>
<client>.kpnxchange.com
Remote Server returned '550 5.0.0 Mail rejected.'