554 validating sender
This may occur if the sender domain is known as a spam domain.To check if a domain is listed, you may use a lookup tool such as the following: Spamhaus.Please follow our email policy in order to ensure a trouble-free email communication.
Unless you simply made a typo in the email address, this is not something that you can fix, and must be addressed at the other end.
Please contact your administrator who should correct the server configuration based on our The specified recipient does not exist on our systems or the recipient has not used their inbox for an extended period of time and the inbox has been temporarily disabled due to inactivity. The e-mail was rejected since it violates our guidelines.